The Appendages of Ptychoparia.

Fig. 11.â€”Triarthrus beckiGreen. Anal plate of specimen 65525 in the U. S. National Museum. Drawn by Doctor Wood. Ã— 20.

The anal plate is especially well shown by specimen 65525 in the United States National Museum (fig. 11). This specimen is from Rome, New York, and two photographs of it have been published by Walcott (1918, pl. 29, fig. 6; pl. 30, fig. 19). It is developed from the dorsal side, and the anal plate is displaced, so that it projects behind the end of the pygidium. It is semicircular in shape, with a hemispheric mound at the middle of the anterior half. Two furrows starting from the anterior edge on either side of the mound border its sides, and, uniting back of it, continue as an axial furrow to the posterior margin. The mound is perforated for the opening of the posterior end of the alimentary canal. The lateral borders of the plate bear five pairs of short, symmetrically placed spines. The plate is 1 mm. wide and 0.5 mm. long, and the entire trilobite is 11.5 mm. long.

Ptychoparia striata(Emmrich).

Illustrated: Jaekel, Zeits. d. d. geol. Gesell., 1901, vol. 53, part 1, pls. 4, 5.

Jaekel has described a specimen of this species obtained from the Middle Cambrian near Tejrovic, Bohemia, which on development showed beneath the test of the axial lobe, certain structures which he believed represented the casts of proximal segments of appendages. On the basis of this specimen he produced a new restoration of the ventral surface of the trilobite, in which he showed three short wide segments in the place occupied by the coxopodite of an appendage ofTriarthrus. He also made the mouth parts considerably different from those of the latter genus. Beecher (1902) showed that the structures which Jaekel took for segments of appendages were really the fillings between stiffening plates of chitin on the ventral membrane, and demonstrated the fact that similar structures existed inTriarthrus. It cannot be said, therefore, that any appendages are really known inPtychoparia striata, but some knowledge of the internal anatomy of the species is supplied by the specimen.

Ptychoparia cordillerÃ¦(Rominger).

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192, pl. 24, fig. 2;â€”Ibid., vol. 67, 1918, pl. 21, figs. 3-5 (corrected figure).

Walcott has figured a single individual of this species showing appendages, the accompanying description being as follows (1918, p. 144):

Ventral appendages. Only one specimen has been found showing the thoracic limbs. This indicates very clearly the general character of the exopodite and that it is situated above the endopodite, although there are only imperfect traces of the latterâ€¦.The exopodites are unlike those of any trilobite now known. They are long, rather broad lobes extending from the line of the union of the mesosternites and the pleurosternites. At the proximal end they appear to be as wide as the axial lobe of each segment, and to increase in width and slightly overlap each other nearly out to the distal extremityâ€¦. They are finely crenulated along both the anterior and dorsal margins, which indicates the presence of fine setÃ¦.

The exopodites are unlike those of any trilobite now known. They are long, rather broad lobes extending from the line of the union of the mesosternites and the pleurosternites. At the proximal end they appear to be as wide as the axial lobe of each segment, and to increase in width and slightly overlap each other nearly out to the distal extremityâ€¦. They are finely crenulated along both the anterior and dorsal margins, which indicates the presence of fine setÃ¦.

The specimen is quite imperfectly preserved, but seems to indicate that the exopodite of Ptychoparia had a long, rather narrow unsegmented shaft.

Measurements(from Walcott's figure): The specimen is a small one, about 9.5 mm. long, an individual exopodite is about 2 mm. long and the shaft 0.33 mm. wide.

Horizon and locality:Middle Cambrian, Burgess shale, between Mount Field and Wapta Peak, above Field, British Columbia.

Ptychoparia permultaWalcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, p. 145, pl. 21, figs. 1, 2.

Walcott figured one individual of this species showing long slender antennules projecting in front of the cephalon. It is of especial interest because one of the antennules shows almost exactly the same sigmoid curvature which is so characteristic of the relatedTriarthrus. The individual segments are not visible.

Measurements:The specimen is 23 mm. long and the direct distance from the front of the head to the anterior end of the more perfect antennule is 9.5 mm. Measured along the curvature, the same antennule is about 11 mm. long.

Horizon and locality:Same as the preceding.

Kootenia dawsoniWalcott.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 14, figs. 2, 3.

One specimen figured by Doctor Walcott shows the distal ends of some of the exopodites and endopodites of the right side. He compares the exopodites with those of Neolenus, stating that the shaft consists of two segments, the proximal section being long and flat, fringed with long setÃ¦, while the distal segment has short fine setÃ¦. The endopodite best shown is very slender, and the segments are of uniform width and only slightly longer than wide.

Measurements (from Walcott's figures): Length of specimen, about 41 mm. Length of five distal segments of an endopodite, 7.5 mm. Since the pleural lobe is only 7 mm. wide, the endopodites, and probably the exopodites also, must have projected a few millimeters beyond the dorsal test when extended straight out laterally.

Formation and locality: Burgess shale, Middle Cambrian, on the west slope of the ridge between Mount Field and Wapta Peak, above Field, British Columbia.

HISTORICAL.

All of the work on these species has been done by Doctor Walcott, who summarized his results in 1881.

In the first of his papers (1875, p. 159), Walcott did not describe any appendages but paved the way for further work by a detailed and accurate description of the ventral surface of the dorsal shell of Ceraurus. He demonstrated the presence in this species of strongly buttressed processes which extend directly downward from the test just within the line of the dorsal furrows. One pair of these is seen beneath each pair of the glabellar furrows, each segment of the thorax has a pair, and there are four pairs on the pygidium. He pointed out also that these projections were but poorly developed on that part of the glabella which is covered by the hypostoma. He called them axial processes, the only name which appears to have been suggested thus far.

The first announcement of the discovery of actual appendages inCeraurusandCalymenewas made by the same investigator in a pamphlet published in 1876 in advance of the 28th Report of the New York State Museum of Natural History, the publication of the whole report being delayed till 1879. The results were obtained by the process of cutting translucent slices of enrolled trilobites derived from the Trenton limestone at Trenton Falls, New York. Since he summarized all the results of this study in one paper at a later date, it is not necessary to follow the stages of the work.

A second preliminary paper was published in pamphlet form in September, 1877, and in final form in 1879, when the first figures were presented.

In his important paper of 1881, Walcott reviewed all that was known of the appendages of trilobites to that time, and gave the results of seven years of study of sections of enrolled specimens. Slices had been made of 2,200 individuals from Trenton Falls, which resulted in obtaining 270 which were worthy of study. Of these, 205 were fromCeraurus pleurexanthemus, 49 fromCalymene senaria, 11 fromIsotelus gigas, and 5 fromAcidaspis trentonensis.

Walcott's views on certain portions of the anatomy can best be set forth in the form of a few extracts (1881, pp. 199-208):

The Ventral Membrane.â€”In those longitudinal sections in which the ventral membrane is most perfectly preserved, it is shown to have been a thin, delicate pellicle or membrane, strengthened in each segment by a transverse arch, to which the appendages were attached. These arches appear as flat bands separated by a thin connecting membrane, somewhat as the arches in the ventral surface of some of the Macrouran Decapodsâ€¦.

In by far the greater number of sections, both transverse and longitudinal, the evidence of the former presence of an exterior membrane, protecting the contents of the visceral cavity, rests on the fact that the sections show a definite boundary line between the white calcspar, filling the space formerly occupied by the viscera, and the dark limestone matrix. Even the thickened arches are rarely seen.

The mode of attachment of the leg to the ventral surface is shown [in transverse and longitudinal sections ofCeraurusandCalymene]. These illustrations are considered as showing that the point of articulation was a small, round process projecting from the posterior surface of the large basal joint, and articulating in the ventral arch somewhat as the legs of some of the Isopods articulate with the arches in the ventral membrane. The arches of the ventral membrane in the trilobite â€¦ afford a correspondingly firm basis for the attachment of the legs.

Branchial appendages.â€”The branchiÃ¦ have required more time and labor to determine their true structure than any of the appendages yet discovered. They were first regarded as small tubes arranged side by side, like the teeth in a rake; then as setiferous appendages, and finally as elongate ribbon-like spirals and bands attached to the side of the thoracic cavity, the epipodite being a so-called branchial arm. All of these parts are now known to belong to the respiratory system, but from their somewhat complex structure, and the various curious forms assumed by the parts when broken up and distorted, it was a long time before their relations were determined.

The respiratory system is formed of two series of appendages, as found beneath the thorax. The first is a series of branchiÃ¦ attached to the basal joints of the legs, and the second, the branchial arms, or epipodites.

The branchiÃ¦, as found inCalymene,Ceraurus, andAcidaspis, have three forms. In the first they bifurcate a short distance from the attachment to the basal joint of the leg, and extend outward and downward as two simple, slender tubes, or ribbon-like filaments. In the second form they bifurcate in the same mariner, but the two branches are spirals. These two forms occur in the same individual but, as a rule, the more simple ribbon-like branchia is found in the smaller or younger specimens, and the spiral form in the adultâ€¦. The spiral branchiÃ¦ of Ceraurus are usually larger and coarser than those ofCalymene.

The third type of the branchiÃ¦ [consists of rather long straight ribbons arranged in a digitate manner on a broad basal joint]. As far as yet known, this is confined to the anterior segments of the thorax.

The epipodite or branchial arm was attached to the basal joints of the thoracic legs and formed of two or more joints. This has been called a branchial arm, not that it carried a branchia, but on account of its relation to the respiratory system. It is regarded as an arm or paddle, that, kept in constant motion, produced a current of water circulating among the branchiÃ¦ gathered close beneath the dorsal shell. . . .

Of the modification the respiratory apparatus underwent beneath the pygidium, we have no evidence.

In his latest publication (1918, pp. 147-153, pls. 26-28, 33), Walcott has reviewed his earlier work onCalymeneandCeraurus, and presented a new restoration of the former. The coxopodites are now interpreted as being similar to those ofTriarthrusand Neolenus, but the exopodites are still held to be spiral and the setiferous organs labelled as epipodites rather than exopodites.

As one may see by reading the above quotations from Doctor Walcott's descriptions, he found certain branchial organs inCeraurusandCalymenewhich have not been found in other trilobites but otherwise the essential features of the appendages of all are in agreement.

Spiral BranchiÃ¦.

It is now necessary to inquire if the thin sections can not be interpreted on the basis of trilobites with the same organs asTriarthrus. The interpretation of the structures seen in these translucent slices is exceedingly difficult, and Doctor Walcott deserves the utmost praise for the acumen with which he drew his deductions. Even with the present knowledge ofTriarthrus,Isotelus, andNeolenusas a guide, I do not think it is safe to speak dogmatically about what one sees in them.

Walcott has summarized his results in his restoration of the appendages ofCalymene(1918, pl. 33). The coxopodite supports a slender six-jointed endopodite as inTriarthrus, dorsal to which is a short setiferous epipodite which differs from the exopodite ofTriarthrus, in being less long, unsegmented, and in having shorter setÃ¦. Arising from the same part of the coxopodite with this epipodite is the bifurcate spiral branchia which has not been seen in this form in other trilobites. The evidence on which the existence of this organ is postulated consists of a series of sections across the thorax, the best of them figured by Walcott in his plates 2 and 3 (1881) and plate 27 (1918).

The specimens sliced were all partially or quite enrolled, and in that position one would expect to find the appendages so displaced that it would be only rarely that a section would be cut, either by chance or design, in such a direction as to show any considerable part of any one appendage. This expectation has proved true in regard to the endopodites, the sections rarely showing more than two or three consecutive segments. Sections like those shown in figures 1 and 2 in plate 2 (1881) seem to be unique. On the other hand, there are numerous slices showing the so-called spiral branchiÃ¦. They show for the most part as a succession of rectangular to kidney-shaped spots of clear calcite.[1]Usually these clear spots are isolated, not confluent, but in a small number of specimens, perhaps three or four, the spots are connected in such a way as to show a zig-zag band which suggests a spiral. Such an explanation is of course entirely reasonable, but it would be surprising if so slender a spiral should be cut in such a way as to exhibit the large series of successive turns shown in many of these thin sections. Continuous sections of such organs should be no more common than continuous sections of endopodites.

[1]In looking at Walcott's figures of 1881, it should be remembered that the dark portions of the figures are clear calcite in the specimens, while the light part is the more or less opaque matrix.

One of the arguments against the interpretation of these series of spots as sections across spiral arms is that of probabilities. It is known from flattened specimens thatNeolenus,Kootenia,Ptychoparia,Triarthrus, andCryptolithusall have a single type of exopodite, consisting of a simple setiferous shaft. All these genera have been examined in a way that permits no doubt about the structure, and no trace of spiral arms has been detected. On the other hand, Walcott found spiral arms in three unrelated genera,Calymene,Ceraurus, andAcidaspis, all of the trilobites in which he found exopodites by the method of sectioning. What are the probabilities that genera of three different families, studied by means of sections, should agree in having a type of exopodite different from that of the five genera about whose interpretation there can be no doubt?

Another argument against the interpretation of the sections as spirals is that in any one line the individual spots are of roughly uniform size. This means of course that the spiral has been cut by a plane parallel to the tangent plane. This might happen once, just as once Doctor Walcott cut all six segments of a single endopodite, but that it should happenrepeatedly is highly improbable. Moreover, there is a limit to the diameter of the section which may be made from these slender spirals. Most of the spots have one diameter about one half greater than the other, but others are from three to six times as long as wide. These last could obviously be cut only from a very large spiral, and they are therefore interpreted by Walcott as setÃ¦ of epipodites. Yet all gradations are found among the sections, from the long setÃ¦ to the short dots. (See pl. 27, 1918.) In referring to one slice, Walcott says (1918, p. 152):

In the latter figure and in figure 13, plate 27, the setÃ¦ of several epipodites appear to have been cut across so as to give the effect of long rows of setÃ¦. The same condition occurs in specimens ofMarrellawhen the setÃ¦ of several exopodites are matted against each other.

Fig. 12.â€”A slice ofCeraurus pleurexanthemusin which the exopodite happened to be cut in such a way as to show a part of the shaft and some of the setÃ¦ in longitudinal section. Specimen 80. Ã— 4.

This is certainly an apt comparison, and equally true ifNeolenus,Triarthrus, orCryptolithuswere substituted forMarrella.

Now consider the "epipodites." They are well shown inCalymenein the specimens illustrated on plate 27, figure 11 (1918), and plate 3, figure 3 (1881), and less clearly in one or two others. Slices 22 (pl. 27, fig. 12, 1918) and 80 (ourfig. 12) show what is called the same organ in Ceraurus. It will be noted that all of these slices are cut in the same way, that is, more or less parallel to the under surface of the head, or, at any rate, on a plane parallel to a plane which would be tangent to the axial portion of the coiled shell. The sections which show the spirals best are those which are cut by a plane perpendicular to the long axis of the body. If one were to attempt to cut an enrolledTriarthrusin such a way as to get a section showing the length of the setÃ¦, one would not cut a section perpendicular to the axis of the animal, nor, in fact, would he cut one parallel to the ventral plane, but it is obvious that in this latter type of section he would stand a better chance of finding a part of the plane of the exopodite coincident with the plane of his section than in the former. And that seems to be what has happened in these sections ofCalymeneandCeraurus. If the exopodites were preserved, transverse sections were bound to cut across many sets of fringes, and the resultant slice would show transverse sections of the setÃ¦ as a series of overlapping spots. A few fortunately located sections in a more nearly horizontalplane might cut the setÃ¦ and occasionally the shaft of one or more exopodites in the longitudinal plane, and the resulting effect would produce the so-called "epipodites." A careful study has shown that no one of these epipodites is complete, and they do not have the palmate form shown in Walcott's figures.

And the last and most important argument against the spiral appendages is that certain slices, of bothCalymeneandCeraurus, show definitely exopodites of exactly the type found in other trilobites. These are discussed later in the detailed description of the various slices.

If these series of spots are interpreted on the basis of the known structure ofTriarthrus, they are of course a series of sections through the setÃ¦ of the exopodites. It will be shown in Part IV that these setÃ¦ are not circular in section, but flattened, inCryptolithuseven blade-like, and that they overlap one another. A section across them would give the same general appearance as, for instance, that shown in figures 4, 6, 9, and 10 of Walcott's plate 3 (1881).

When both endopodites and the "spiral branchiÃ¦" are present in the same section (pl. 1, fig. 4; pl. 2, figs. 1, 2), the "spiral branchiÃ¦" are dorsal to the endopodites, as the setÃ¦ of the exopodites would be expected to be. The specimens which show the clear spots connected, and which suggest a spiral (pl. 3, fig. 5), may seem at first sight to bear evidence against this interpretation, but one has only to think of the effect of cutting a section along the edge where the setÃ¦ are attached to the shaft of the exopodite ofTriarthrusto see that such a zig-zag effect is entirely possible. One would expect to cut just this position only rarely, and, in fact, the zig-zags are seen in only three or four sections. The bifurcation of the basal segment of the "spiral branchiÃ¦" (pl. 3, fig. 10, 1881) is probably more apparent than real, if indeed these basal segments have anything to do with the succeeding one.

A second peculiarity ofCalymene, shown in Walcott's restoration, is the great enlargement of the coxopodites and of the distal segments of the endopodites of the fifth pair of appendages of the cephalon. This is based on the sections of plate 3, figures 6, 7, 8, 9, 10 (1881). After a study of the specimens I regret to find myself still unconvinced that the posterior cephalic appendages were any larger than those in front.

Ventral Membrane.

The most striking value of the thin sections ofCeraurusandCalymene, and therein they have a great superiority over all the other forms so far investigated, is that they show the extent of the body cavity and the position, though not the substance, of the ventral membrane. Transverse sections throughCeraurus(Walcott's pl. 1. figs. 1-5; pl. 2, figs. 1, 3, 1881) andCalymene(pl. 3, figs. 9, 10, 1881) show that the body cavity was almost entirely confined to the axial lobe. The longitudinal sections ofCeraurus(pl. 2, figs. 6, 8; pl. 4, fig. 8) and ofCalymene(pl. 2, figs. 5, 7; pl. 5, figs. 1-4) show that the ventral membrane was exceedingly thin and was wrinkled transversely when the shell was enrolled.

The specimens of figures 1-3, plate 5 (1881) show the form of the ventral membrane more distinctly than any of the others. The section of figure 1 was cut just inside the dorsal furrow on the right side, and figure 2, which is on the opposite side of the same slice, is almost exactly on the median line. Figure 3 shows a section just inside the left dorsal furrow. Section 2 did not cut any of the appendages, and the ventral membrane isshown as a thickened, probably chitinous sheet thrown into low sharply crested folds equal in number to, and pointing in a direction just the reverse of, the crests of the segments of the thorax. Under the pygidium, where there would of course be less wrinkling, the folds are hardly noticeable. In the actual specimens one sees more plainly than in the figures the line of separation between the ventral membrane and the appendages, but the state of preservation of everything beneath the dorsal shell is so indefinite that one does not feel sure just what the connection between the appendages and the membrane was. In the original of figure 5, plate 2, which seems to have been cut so as to cross the appendages at their line of junction with the ventral membrane, there appear to be narrow chitinous (?) plates extending from the ventral membrane to the dorsal test.

Appendifers.

In Ceraurus there are regular calcareous processes which extend down from the dorsal test just inside the line of the dorsal furrow, and which undoubtedly serve as points of attachment of the appendages. These processes, which for convenience I have designated as "appendifers," are broken off in most specimens showing the lower surface ofCeraurus pleurexanthemus, but on certain ones cleaned with potash they are well preserved. Doctor Walcott showed them well in his figures of the lower surface of this species (1875, pl. 11; 1881, pl. 4, fig. 5), while the attempt of Raymond and Barton (1913, pl. 2, fig. 7) to show them by photography was not so successful.

There is one pair of appendifers on each of the thoracic segments and four pairs on the pygidium. On the cephalon there is one pair under the neck furrow, and a pair under the posterior glabellar furrows. These are not concealed by the hypostoma. Further forward, and completely covered by the hypostoma, are two much less strongly developed but similar ones, so that there are in all four pairs of appendifers on the cephalon, though it is extremely doubtful if the appendages were articulated directly to all of them. On a specimen ofCeraurus pleurexanthemus30 mm. long on the median line, the dorsal furrows are 7.5 mm. apart at the anterior end of the thorax, and the tips of the appendifers of this segment are only 4 mm. apart. Each consists of a straight slender rod with a knoblike end projecting directly downward from the dorsal test, and supported by a thin calcareous plate which runs diagonally forward to the anterior edge of the segment directly under the dorsal furrow. On the pygidium three pairs of the appendifers have this form, while the fourth pair consist of low rounded tubercles which are concealed by the doublure. These appendifers are probably cut in many of Walcott's sections of Ceraurus, but owing to the state of preservation it is not always possible to determine what part is appendage, what part is body cavity, and what part is appendifer.

Nearly forty years ago Von Koenen (1880, p. 431, pl. 8, figs. 9, 10) described and figured the appendifers of Phacops latifrons. He found them to be calcareous projections on the hinder margin of each segment, converging inward, and about 1.5 mm. long. He correctly considered them as supports (StÃ¼tzpunkte) for the feet.

Appendifers are well developed also in Pliomerops, and in well preserved specimens ofCalymene senariafrom Trenton Falls they are present, but instead of being rod-like processes, they are rather thick, prominent folds of the shell. They are also well shown in some of the thin sections. A specimen ofTriarthrus(No. 229, ourpl. 5, fig. 2) has broad processes extending downward from the lower side of the test below the dorsal furrows, much as inCalymene, and the individual ofCryptolithusshown inplate 8, figure 1,possesses slender appendifers. Two other specimens (Nos. 237 and 242) show them quite well. They were probably present in all trilobites, but seldom preserved. The appendifers have the same origin as the entopophyses ofLimulus, and like them, may have relatively little effect on the dorsal surface.

Calymene senariaConrad.

(Text figs.13-16,23.)

Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pl. 1, figs. 6-10; pl. 2, figs. 5-7, 10; pl. 3, figs. 1, 3, 8-10; pl. 4, figs. 3, 7; pl. 5, figs. 1-6; pl. 6, figs. 1 (restoration), 2;â€”Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1. fig. 7 (restoration);â€”Geol. Mag., dec. 4, vol. 1. 1894, pl. 8, figs. 7, 8;â€”Smithson. Misc. Coll., vol. 67, 1918, pl. 26, figs. 1-7, 9-13; pl. 27, figs. 4, 5 (not 5a), 11 (not 12,Ceraurus), 13, 14, 15 (notCeraurus); pl. 28, figs. 7, 8; pl. 33, fig. 1 (restoration); pl. 34, fig. 2; pl. 35, fig. 6.â€”Dames, N. Jahrb. f. Min., etc., vol. 1, 1880, pl. 8, figs. 1-5.â€”Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 11, figs. 19-32; pl. 12, figs. 33-41.â€”Packard, Amer. Nat., vol. 16, 1882, p. 796, fig. 12.â€”Bernard, The ApodidÃ¦, 1892, text figs. 50, 52, 54;â€”Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 13, 15, 17.â€”Å’hlert, Bull. Soc. GÃ©ol. France, ser. 3, vol. 24, 1896, fig. 12.â€”Beecher, Amer. Jour. Sci., vol. 13, 1902, pl. 5, fig. 7.

In both of Walcott's accounts (1881, 1918) of the appendages ofCalymeneandCeraurus, he has described them together, so that those who have not taken time to study the illustrations and disentangle the descriptions are very apt to have a confused notion in regard to them. I have therefore selected from the original specimens those slices ofCalymenewhich are most instructive, and bearing in mind the probable appearance of the appendages of an enrolledTriarthrus, have tried to interpret them. In such a method of study, I have of course started with a pre-formed theory of what to expect, but have tried to look for differences as well as likenesses.

Cephalic Appendages.

Antennules.â€”The evidence of antennules rests on a single slice (No. 78). The appendage in question is exceedingly slender and arises at the side of the hypostoma near its posterior end. It shows fine, slender segments, and curves first outward and then forward. If it is in its natural position, it is not an antennule, but the endopodite of the second or third pair of cephalic appendages. It is short, only about one-third the length of the hypostoma, but is doubtless incomplete. The two distal segments show a darker filling, indicating that they were hollow. Judging from analogy with other trilobites, the appendage is probably an endopodite and not an antennule. There can be no reasonable doubt, however, thatCalymenepossessed antennules.

Some idea of the form of the coxopodites of the cephalic appendages may be obtained from sections which cut in approximately the plane of the hypostoma. Such sections are shown in Walcott's photographs (pl. 26, figs. 4, 6, 11, 1918). Specimens 50 (fig. 4, ourfig. 13), 51 (fig. 6), 6 (fig. 11), and 40 (ourfig. 14) agree in showing two pairs of slender coxopodites which are attached at the sides of the hypostoma and run backward parallel and close to it, and two pairs of larger coxopodites which are behind the hypostoma, although the point of attachment of the third pair is in front of its tip. The anterior pair are apparently under-developed and no longer function as mouth parts, while the posterior two pairs are large and armed on their inner ends with spines. Specimen 78, which has already been mentioned in connection with the antennules, shows a second very slender appendage back of the so called antennule, which is equally slender, but is directed outward instead of forward.It seems not improbable, from their position and similarity, that these two are the endopodites of the first two appendages on one side of the hypostoma. Specimen 6 shows rather inadequately the endopodites of the second and third cephalic appendages. I have not found other slices showing endopodites of the cephalon. Walcott, in both his restorations, has shown enlarged, paddle-shaped dactylopodites on the distal ends of the fourth cephalic endopodites. The evidence for this rests principally on three slices, No. 38 (pl. 26, figs. 9, 10), 53 (pl. 26, fig. 12), and 43 (pl. 26, fig. 13). Of these, No. 43 may be dismissed at once as too poorly preserved to be interpreted. No. 53 does show a section of an appendage which seems to have an unusually wide dactylopodite, but this slice presents no evidence at all as to the appendage to which the dactylopodite appertains, nor can one even be sure that there has not been a secondary enlargement. Specimen 43 shows this feature much less definitely than is indicated by the published photograph and drawing. The segment in question is strongly curved, with a constriction possibly dividing it into two. If it is in its natural position in this section, it obviously belongs to one of the thoracic segments and not to the cephalon. With evidence of difference so unsatisfactory, I prefer to reconstruct the posterior cephalic endopodites on the same plan as those of the thorax.

Fig. 13.â€”Slice throughCalymene senariain the plane of the hypostoma, showing the very slender coxopodites beside that organ, the spines on the inner end of one of the maxillulÃ¦, and the anterior position of the attachment of all these appendages. From a photographic enlargement. Specimen 50. Ã— 4.

Fig. 14.â€”Slice through the hypostoma and thorax ofCalymene senariaConrad, showing the small size of the coxopodites nearest the hypostoma. Shell in black, appendages and filling of abdominal cavity dotted. From a photographic enlargement. Specimen 40. Ã— 3.8.

Fig. 15.â€”Transverse section ofCalymene, showing method of articulation with the appendifer. The shell is in solid black, the filling of the appendage and appendifer stippled. Traced from a photographic enlargement of the slice. Specimen 63. Ã— 7.

Exopodites.â€”Walcott admits that there is no direct evidence of spiral exopodites in the cephalon ofCalymene. No one of the sections cutting through the plane of the hypostoma shows any trace of appendages which could be interpreted as exopodites.

Thoracic Appendages.

The large coxopodites of the anterior thoracic appendages are well shown in many specimens cut longitudinally, of which Nos. 23, 50, and 55 may be mentioned, since photographs of them have been published by Walcott (pl. 26, figs. 1-4, 1918). The endobases of all taper toward the proximal ends. Transverse slices show sections of the coxopodites whichare no wider than those in longitudinal sections, indicating that they were not compressed but probably cylindrical. This is borne out by an individual (pl. 28, fig. 7, 1918) which is not a slice but an actual specimen, the body cavity of which was hollow, and, opened from above, shows the impressions of the last two coxopodites of the cephalon, and the first four of the thorax.

One transverse section (No. 63, see our fig. 15) is especially valuable, as it shows the method of articulation of the coxopodites with the dorsal skeleton. Another specimen (No. 73) shows that appendifers are present inCalymene, and while the appendifer does not retain its original form in slice No. 63, the section does show clearly that there was a notch in the inner (upper) side of the coxopodite into which the lower end of the appendifer fitted, thus giving a firm, articulated support for the appendage. This notch appears to be slightly nearer the outer than the inner end of the coxopodite, and since it must have made a kind of ball-and-socket joint, considerable freedom of movement was allowed. The appendage must have been held in place by muscles within the coxopodite and attached to the appendifer.

No slice which I have seen shows a continuous section through all the segments of an endopodite, but many, both longitudinal and transverse, show one, two, or as many as three segments.

Such sections as No. 120 show that the endopodites of the thorax were slender and composed of segments of rather uniform diameter. Other sections, notably No. 83, 154, and in, show that they tapered distally, and bore small spines at the outer end of each segment.

The exopodites of course furnish the chief difficulty in interpretation. Doctor Walcott finds two sets of structures attached to the coxopodite, a long, slender, spiral exopodite, and a short, broad epipodite with a fringe of long setÃ¦. Since he has given the same interpretation forCalymene,Ceraurus, andAcidaspis, I have considered the question of all three together on a preceding page (p. 48), and given my reasons for regarding both structures as due to sections in different directions across setiferous exopodites.

Sections like those shown in figures 11, 13, and 14 of plate 27 (1918) happen to be cut in or near the plane of the setÃ¦ of an exopodite, and so show hairs of considerable length. Such sections are, as would be expected, very few in number, while sections like those shown on figures 4, 5, 7, and 9 of plate 27, which cut the setÃ¦ more nearly at right angles, are very common. Slices which give any definite idea of the form of the shaft of the exopodite are exceedingly rare. Perhaps the most satisfactory one is No. 23 (pl. 3, fig. 3, 1881), which shows the proximal part of a long, slender, unsegmented shaft, with the bases of a number of slender setÃ¦. The organ is not complete, as would be inferred from the published figure, but the section cuts diagonally across it, and the total length is unknown. It is directed forward, like the exopodites of Neolenus, but whether or not this is a natural position is yet to be learned.

The proximal, non-setiferous portion of the exopodite is evidently at an angle with the setiferous part. Another similar exopodite is apparently shown by specimen 29 (pl. 3, fig. 9, 1881), which has a similar angulated shaft and just a trace of the bases of the setÃ¦.

Pygidial Appendages.

That appendages were present under the pygidium is shown by longitudinal sections, but nothing is known of the detail of structure.

Fig. 16.Restoration ofCalymene senariaConrad, based upon data obtained from the study of the translucent sections made by Doctor Walcott. Prepared by Doctor Elvira Wood, under the supervision of the author. About twice natural size.

Relation of Hypostoma to Cephalon in Calymene.

InCalymenethe shape of the hypostoma bears little relation to the shape of the glabella, and it is relatively smaller, both shorter and narrower, than in Ceraurus. In shape, neglecting the side lappets at the front, it is somewhat rectangular, but rounded at the back, where it is bifurcated by a shallow notch. The anterior edge has a narrow flange all across, which is turned at almost right angles to the plane of the appendage, and which fits against the doublure of the free cheeks at the sides and against the epistoma in the middle. The side lappets show on their inner (upper) surface shallow pits, one on each lappet, which fit over projections that on the dorsal surface show as deep pits in the bottom of the dorsal furrows in front of the anterior glabellar furrows. The appendifers on the head inCalymenetake the form of curving projections of shell underneath the glabellar and neck furrows, and owing to the narrowness of the hypostoma, all these are visible from the ventral side, even with it in position. This shield extends back about 0.6 ofthe length of the cephalon, and to a point a little behind the second glabellar furrow from the back of the head.

In Doctor Walcott's restoration ofCalymenehe has represented all four pairs of biramous appendages as articulating back of the posterior end of the hypostoma. I think his sections indicate that the gnathobases of two pairs of these appendages rested alongside or beneath it, and in particular, the longitudinal sections (1881, pl. 5) would appear to show that the mouth was some distance in advance of its posterior end.

Restoration of Calymene.

(Textfig. 16.)

From what has been said above, it is evident that for a restoration of the appendages ofCalymeneconsiderable dependence must be placed upon analogy with other trilobites. Nothing is positively known of the antennules, the exopodites of the cephalon, or any appendages, other than coxopodites, of the pygidium, but all were probably present. It is inferred from the slices that the first two pairs of cephalic appendages were poorly developed, the endopodites short and very slender, the coxopodites lying parallel to the sides of the hypostoma and nearly or quite functionless. The gnathites of the last two pairs of cephalic appendages are large, closely approximated at their inner ends, and bear small tooth-like spines. The endopodites are probably somewhat better developed than the anterior ones and more like those on the thorax.

The coxopodites of the thorax appear to have had nearly cylindrical endobases which tapered inward. The endopodites were slender, tapering gradually outward, and probably did not extend beyond the dorsal test. Small spines were present on the distal end of each segment. Each exopodite had a long, slender, unsegmented shaft, to which were attached numerous long, overlapping, flattened setÃ¦. The shaft may have been angulated near the proximal end, and may have been directed somewhat forward and outward as in Neolenus, but the evidence on this point is unsatisfactory. The number of pairs of appendages is that determined by Walcott from longitudinal sections, namely, four pairs on the cephalon beside the antennules, thirteen pairs in the thorax, and nine pairs on the pygidium.

Calymenesp. ind.

(pl.6, figs.4,5.)

Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pl. 6, figs. 5a, b;â€”Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1, fig. 10;â€”Geol. Mag., dec. 4, vol. 1, 1894, pl. 8, fig. 10;â€”Smithson. Misc. Coll., vol. 67, 1918, pl. 36, figs. 1, 2, 2a-d.â€”Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12, 1881; pl. 12, figs. 44a, b.

In the United States National Museum there is a thin piece of limestone, about 3 inches square, which has on its surface eight jointed objects that have been called legs of trilobites. Two of these were figured by Walcott (1881, pl. 6, fig. 5). The slab contains specimens ofDalmanellaandCryptolithus, in addition to the appendages of trilobites, and is said by Doctor Ulrich to have come from the tipper part of the Point Pleasant formation (Trenton) on the bank of the Ohio River below Covington, Kentucky.

The specimens are all endopodites of long slender form, similar to those ofTriarthrus, but since that genus does not occur in the Point Pleasant, it is necessary to look upon some other trilobite as the former possessor of these organs. BothIsotelusandCalymeneoccurat this horizon, and as the specimens obviously do not belong toIsotelusorCryptolithus, it is probable that they were formerly part of aCalymene.

All the endopodites are of chitinous material, and the various specimens show, according to the perfection of their preservation, from four to six segments. The endopodite as a whole tapers but slightly outward, and the individual segments are of nearly equal length. They appear to be but little crushed, and are oval in section, with a crimped anterior and posterior margin. One or two show a median longitudinal ridge, such as is seen in some appendages ofTriarthrus. Each segment is parallel-sided, with a slight expansion at the distal end, where the next segment fits into it.

Under the heading "Ordovician Crustacean Leg," Walcott (1918, p. 154, pl. 36, figs. 1,2) has recently redescribed these specimens, and thinks that they do not belong toCalymene, nor, indeed, to any trilobite. He concludes that they were more like what one would expect in an isopod. Passing over the fact that the oldest isopod now known is Devonian, the fossils in question seem to me quite trilobite-like. Walcott says:

The legs are associated with fragments ofCalymene meekibut it is not probable that they belong to that species; if they did, they are unlike any trilobite leg known to me. The very short coxopodite and basopodite are unknown in the trilobites of which we have the legs, as they are fused into one joint forming the long protopodite in the trilobite. The distal joint is also unlike that of the trilobite legs known to us.

A great deal of Doctor Walcott's difficulty probably arises from his homology of the coxopodite of the trilobite with the protopodite of the higher Crustacea. The coxopodite of the trilobite is not fused with the basipodite, this latter segment always remaining free. Indeed, Walcott himself says ofNeolenus(1918, p. 128):

Each thoracic leg (endopodite) is formed of a large elongate proximal joint (protopodite), four strong joints each about 1.5 times as long as wide (basopodite, ischiopodite, meropodite and carpopodite); two slender elongate joints (propodite and dactylopodite) and a claw-like, more or less tripartite termination.

Walcott's drawing (pl. 36, fig. 1) is a composite one, and while it shows eight segments, I was not able to count more than seven on any of the specimens themselves. In regard to the terminal segment, the dactylopodite of the limb shown in his plate 36, figure 2, is unusually long, and a comparison with other photographs published on the same plate shows that such long segments are unusual.

Proof that these are appendages of aCalymeneis of course wanting, but there is no particular reason so far to say that they are not.

Measurements:Two of the more complete specimens, each showing six segments, are each 8 mm. long.

Somewhat similar to the specimens from Covington are the ones described by Eichwald (1825, p. 39, 1860, pl. 21), the specimens being from the Silurian of Gotland. The figure copied by Walcott (1881, pl. 6, fig. 4) has never been looked upon as entirely satisfactory evidence of the nature of the specimen, and so far as I know, the fossil has not been seen by any modern investigator.

Ceraurus pleurexanthemusGreen.

(pl.11; text figs.12,17-19,21,22,24,29,30.)

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