Blind Trilobites.

In the chaos of generic forms and in the great disagreement which prevails as to the systematizing of the trilobites of the Cambrian time, there is a thorough revision of them highly needed by a person having access not only to the literature, but also to the original specimens. It is almost impossible in the present state of things to tell with any degree of certainty how many well established genera had been living during that period. Hence the difficulty of fixing the systematic names of many specimens the visual organs of which are to be described.

My researches on the visual organs of the Cambrian trilobites are founded on the Angelinian Collection in the Swedish State Museum together with collections of foreign species, but also largely on the waste European and American literature, though we have to deplore the often occurring inexactitude of the figures, especially in the older works, and constructed or schematized figures in some of the newer ones, which give a quite false notion of the structure. There is no lack of figures to show how it ought to be, according to preconceived notions and, on the other hand, a great scarcity of representations, to show how it really is. In spite of all this there is a sufficiently great number of well established facts to demonstrate the organization of the Cambrian genera.

The trilobites of this division may be called blind only in so far as they have no eyes on the upper surface of the head, but they may have been provided with visual organs, though more imperfect, on the hypostoma as really seems to have been the case with some of them.

According to the structure of the head shield the blind trilobites may be subdivided into the following well characterized groups. These are:

I. Without facial ridge:1) The Archæan Trilobites.II. With facial ridge:1) The Olenellidæ.2) The Olenidæ and related.

I. Blind trilobites without facial ridge (= »eye lobe»).

Group 1. The Archæan Trilobites.

In these the head shield is in one piece without any facial suture and facial ridge, and without the least trace of anything that might be called a visual organ and they must consequently be considered as totally blind. In contradistinction to the following groups, excepting the oldest Olenellidæ, the head consists of only three parts, 1) glabella, and 2-3) the two fixed cheeks. These genera range from the oldest zone in which hitherto trilobites have been found, that of Olenellus (Holmia) Kjerulfi, to the zone of Paradoxides Forchhammeri, and some, as Agnostus, even continue as high in the Lower Silurian series as in the Brachiopod schists. Beside Agnostus the other genera are Conocoryphe (seven species in the Swedish Cambrium), Toxotis, Ctenocephalus (?), Elyx, Aneuacanthus, Conophrys and Microdiscus.

»Harpides» brevicepsAng., also belongs here. Anopocare ofAngelinshould also be regarded as one of this group. But it cannot be retained any longer because it is founded on two other, well known forms, being, according toLinnarsson, Peltura scarabæoides (pl. 27 fig. 1, a in Pal. Scand.) and young specimens of Sphærophthalmus alatus (ibid. figs. 1 & 2).

It is remarkable that some of the Conocoryphidæ have an imperfect facial ridge, to be compared with the commenced one in SaoBarr.I (pl. 7 fig. 9). So the American Con. trilineata and reticulate.WalcottU. S. Geol. Survey 10th Rep. pt. I, pl. XCV f. 5 & 6. It is, as it were, arrested in the development and these adult trilobites had stopped, where the larva of Sao was proceeding in its second stage. They are the forerunners of the blind trilobites with facial suture, belonging to the third group. It needs scarcely be mentioned that the genera now enumerated have hardly anything in common, beside the general character of the head, and that real affinity exists only between Agnostus and Microdiscus, and probably also between Conocoryphe, Ctenocephalus and Elyx.

Beecher[2]asserts that there is a suture in Agnostus, but in vain we have searched for it in numerous well preserved specimens and DrHolmalso denies its presence.Nor are there any signs of closed up sutures, which also could not possibly be expected in so early a stage of evolution. It may then be taken as well settled that a fundamental character in these the oldest[3]of all known trilobites is the total want of a facial suture and a compactness of the whole head shield which later is broken up in several parts through the disjunction of the free cheeks. In the Lower Silurian formation there are a few genera sharing in the same structure of the head shield, though by no means else related. Such are Dindymene, Areia, Carmon and in the U. Silurian Cromus. The two species formingBarrande'sgenus Dindymene are so dissimilar that Dind. Friderici Augusti had better to be removed to a new genus and the first described one to be retained as type of the genus Dindymene. The same is the case with Carmon, where the type species C. mutilus is blind and without free cheeks while the other species, known only by its fixed cheeks and glabella is one of the Olenidæ.

[2]Nat. Classification of the Trilobites, p. 183.

[3]Oldest in that sense that they are the descendants of an archaic precambrian stock, the chief characteristics of which they have retained in the main unchanged and persisting long ages after the close of the Cambrian times, some, as Agnostus, continuing high up in the Lower Silurian.

II. Blind trilobites with facial ridge.

This large division embraces the second and third groups or, with a few exceptions, all the rest of the Cambrian trilobites on account of a feature in the cephalic sculpture common to them all, though widely different as to its first origin in both. What forms the prominent and common characteristic of these two groups is the presence of thefacial ridge, which emanates from the basis or the front of the foremost segment of the glabella and in a great variety of different shapes continues backwards near to the posterior border of the head. It has received several names aseye-line, palpebral lobe, ocular ridge, eye-lobe, ocular fillet(Matthew). In German it is namedAugen-leiste, in Frenchfilet(Barrande) and in Swedishögonlist.[4]Some authors make a difference between the more narrow part, calling it eye line, and the thicker posterior node, which they name the palpebral lobe proper.

[4]That name is the most current amongst the swedish authors, together with »palpebrallob»;Holmsaysögonlobandfrontallob.

As this peculiar ridge exists before any facial suture has made its appearance and separated the head shield in five parts, viz. the median glabellar part, the two fixed cheeks and the two free cheeks, and as it occurs in genera which never possessed any facial suture, and where no eye ever was formed, it is not adequate to call it an ocular ridge etc. the more so, as it, at least during a long series of genera succeeding each other, has had no connection whatever with any eye. I therefore propose to call itfacial ridge(in swedishfaciallist). It occurs on the head of almost all Cambrian trilobites, excepting the archaic ones, and it is retained in the later Cambrian Peltura, Sphærophthalmus etc., which have real, compound eyes, as well as in a few Lower Silurian genera as Triarthrus, Pliomera,[5]Euloma, in the Upper Silurian Arethusina and Acidaspis and most persisting inHarpes, ranging from the Lower Silurian Lower Red Orthoceratite Limestone into the middle Devonian beds.

[5]Pl. TörnquistiHolm.

It presents itself in the most variable shapes, and as it in fact can be followed through its development in the oldest known species, it is suitable to begin its description together with the characteristics peculiar to the oldest or second group in this large subdivision, that of theOlenellidæ.

Group 1. The Olenellidæ.

In this group we have two families of different age, the older, less developed the Olenellidæ proper, and the younger the Paradoxidæ. The former consists of the genera Olenellus, Holmia, Mesonacis and Schmidtia, and we shall attend to them first. They have no facial suture[6]and consequently a tripartite head shield like the Archaic ones, no eyes, but there is that strongly developed and most characteristic facial ridge. As seen in Olenellus Thompsoni[7]the crescent like ridge starts as a direct outflow from the base of the first segment of the glabella, and is in direct continuation with it as an integral part. It is roundbacked, regularly curved and at its starting point as broad as one of the posterior glabellar segments. It is regularly semilunate, tapers posteriorly and ends near the occipital segment. From the narrow second segment of the glabella, inside the just described larger ridge, a smaller ridge emanates, broader and flatter than the former, slightly curved, and ends between the third and fourth glabellar segment. Already in Holmia Kjerulfi the ridge is modified. There it is only one ridge, the anterior one, nearly as doubled through a shallow groove running along its back. The second one has dwindled away so as to be seen only as a narrow stripe near the occipital segment and ending outside this in a point. The anterior ridge, represented byHolmas consisting of two nearly independent parts, is indeed in one piece, though its dorsal groove sometimes is deep, and it is with its total breadth joint to the first glabellar segment. Along its outer edge, where it lies close to the cheek, a narrow slit runs, and I suppose that it is to be considered as the first indication of the forming of a facial suture, which however does not reach longer than the ridge. As in Paradoxides it is so tight to the cheek, that there has been no place for an eye. It is no accidental break, its edges being too regular and unbroken in all specimens. This ridge is by far much shorter than in Olenellus and terminates opposite the third segment.

[6]BeecherNatural Classification of the Trilobites, p. 191, pretends that in Olenellus and Holmia real sutures »in a condition of symphysis» occur. He seems to deny the facial sutures and to accept as »real sutures» the »internal sutures» described byHolmin Olenellus Kjerulfi. It is highly doubtful if these interior elevated lines are to be regarded as sutures. They are indeed no sutures, but in reality elevated linear ridges, inclosing, as it were, narrow canals. The real sutures known, the facial sutures, never form elevated lines, be it on the outside of the head or on the inside. Probably these lines are derived from some now unknown interior organization and it may be fit to remind of the somewhat similar though more numerous linear canals of the branchiæ on the interior surface of the great head shield of Apus. (ZaddachDe Apodis cancriformis Anatome ... pl. II fig. 1) or whatHuxley(Anatomy Invertebr. Animals p. 281) calls the convoluted »shell gland» in the carapace. A quite different structure is what I suppose to be the incipient facial suture in Holmia.

[7]Walcottin 10th Ann. Rept. U. S. Geol. Survey, pl. 82 f. 2.

Larva of Olenellus asaphoides, chiefly according toFordandWalcott.

1. Hypothetical figure, based on fig. 2. represents a stage preceding the next. There are three pair of pleura and three segments of the rhachis.

2. Copy ofWalcott'sfig. 5 in »Bullet. 1886», pl. XVII. Three pleura at right, two at left, the larger formed through fusion of the two hindmost in fig. 1.

3. Copy ofWalcott'sfig. 6: the second and third pleura have been united into one.

4. Hypothetical figure as a further development, following upon the stage represented in fig. 3. Fusion completed between all posterior plectra (2, 3, 4), thus forming a single large pleuron composed of the three mentioned.

5. Copy ofFord'sfig. 1, pl. IV, Amer. Journ. Science 1877, enlarged to the same size as Walcott's. First sign of the pygidium.

6. Copy ofFord'sfig. 2 (1877) enlarged. The pygidium has been added to the head shield.

7. Copy ofFord'sfig. 3, enlarged.

8. Copy ofFord'sfig. 1 page 251 in Am. Journ. Oct. 1881, enlarged.

9. Copy ofFord'sfig. 2 page 251. Amer. Journ. Science 1881. Slightly enlarged.

10. Copy ofFord'sfigure 5, pl. IV, Amer. Journ. Sci. 1877, slightly enlarged.

AlreadyLinnarssonobserved the second ridge as he tells in a paper where he describes Olen. Kjerulfi for the first time.[8]It has been called an »ornamental» spine, but in the following we shall learn what it really is. This ridge in connection with its spine has not been observed in any other of the Olenellidæ, at least not in their adult stage.

[8]Öfversigt Vet. Ak. Förhandl. 1871, tafl. XVI fig. 1.

Thanks to the discovery of very early larval stages of the American Olenellus asaphoides, whichFord[9]andWalcott[10]have described and figured, we can through combination of these decipher the development and signification of the facial ridges. To facilitate my explanation I here join a series of cuts from the earliest stage to the more developed, with addition of two schematic stages to complete in a certain degree the series ofWalcottandFord.[11]See figuresp. 13.

[9]In American Journ. of Science 1877 p. 265 and 1881 p. 250.[10]Bullet. U. S. Geol. Survey N:o 30, 1886, pl. 17 fig. 5-6.—Tenth Annual Rept. U. S. Geol. Survey, 1888-89, printed 1890, plate 88, fig. 1, 1 a.[11]In his paper on »The larval stages of Trilobites» p. 175Beechergives a new figure (f. 6), original fromFord'scollection, of the larva of Ol. asaphoides, but it is so sketchy that I cannot with certainty make out what it means. I cannot agree with him when he speaks of free cheeks and eyes in these and he is completely wrong when he says that the outer pair of spines belong to the free cheeks etc. (p. 176).

[9]In American Journ. of Science 1877 p. 265 and 1881 p. 250.

[10]Bullet. U. S. Geol. Survey N:o 30, 1886, pl. 17 fig. 5-6.—Tenth Annual Rept. U. S. Geol. Survey, 1888-89, printed 1890, plate 88, fig. 1, 1 a.

[11]In his paper on »The larval stages of Trilobites» p. 175Beechergives a new figure (f. 6), original fromFord'scollection, of the larva of Ol. asaphoides, but it is so sketchy that I cannot with certainty make out what it means. I cannot agree with him when he speaks of free cheeks and eyes in these and he is completely wrong when he says that the outer pair of spines belong to the free cheeks etc. (p. 176).

Thefigure 1has been hypothetically composed as a deduction fromfig. 2, which presupposes an earlier stage of development like that infig. 1, when there existed three or four pair of lateral appendages in the larva. This then consists of a central portion of five segments. The large anterior crescentlike segment does not, however, show any distinction between a central part and lateral appendages, it is nearly as large as the three next taken together and its backwards bent side horns embrace the next two posterior segments and attain with their narrow pointed tips the back of the fourth segment. The central portion consists of five segments, when the somewhat not well definite posterior marginal segment is taken in account. Each of these segments excepting the fifth one has lateral appendages, those of the second and third segment being quite as broad as the central part and bent backwards in a curve ending in a small pointed spine. The lateral appendages of the fourth segment are largest of all, more than double the length of the two next in front, triangular and standing out beyond the posterior border of the shell as a broad spine.

Infig. 2a great change has set in. There is no distinction between the lateral appendages of the third and fourth segments at left. These two have been fused together, they have united, so as to make the left triangular spine look larger than it was originally. The appendages of the right side are still in the same state as before. But this fusion of the lateral appendages also takes place in another direction, as shown in another specimen (fig. 3), so that the second and third appendages on both sides coalesce into one piece. Now it is easy to imagine that at last a complete fusion has set in between all lateral appendages and that instead of the original three on each side, there is only one large piece, reaching beyond the shell as a broad spine, as represented in the hypotheticalfigure 4.

In thefigure 3a progressive change is also seen in the transformation of the first central segment. From occupying the whole foremost space of the shell it has been lessened in size, more distantiated from the anterior border of the shell and rounded off, moreprominent and definite from the lateral appendages, which have become narrow, though of the same length as before. The two first central segments seem to have been united into one.

Between the hypotheticalfigure 4andfig. 5, there is evidently a great lacuna, not yet filled up. In the interval of time the two appendages, which we saw infig. 4, have been much modified, the posterior one having lost so much in bulk, and the anterior being lengthened and stretching out beyond the border of the shell in a narrow spine alongside the posterior one. The central segmented part has now assumed a shape, which on comparing it with the following stages of development makes its true nature evident and that it indeed is what in the adult animal becomes the glabella. The meaning of the previous stages then also is easily understood. The central segmental piece in them is the glabella or we may, asBernardalready has done, call it for rhachis and the side appendages for pleura, as this little larva represents the whole body of the future trilobite, and embodies all its partsin nuce. Through the great changes which these pleura undergo, it results, as we have seen, that two pair vanish, being incorporated with the large fourth pair and that only two rest for a while, the anterior one being the so much renowned frontal-lob or eye lobe and the second one the so called »ornamental spine», which in fact is a compound of the original second third and fourth pleura of the corresponding segments.

It is to be borne in mind that this larva, which represents the future trilobite in its earliest stage, is nothing but the head, or what in the adult takes the place of the head, and especially its dorsal surface and that it thus solely consists of the future head.

In thefigure 5(Ford'sfig. 1.) there are the first signs of the pygidium coming and in thefig. 6it is well developed, but the thorax is still non apparent. In the stagefig. 6both pleura have increased in length and the compound one also in bulk. They project with spiny points beside the beginning pygidium and the anterior pleura have united across the first segment of the glabella through a narrow ridge, which seems to cut that segment in two. The sequence in the order of development or growth thus is first the head, then the pygidium and last the thorax. At least it is so in these the oldest of all trilobites. But in nearly all trilobites of which there are good data, the head is the part first developed.

Evidently a large hiatus exists between the stages represented in figs.6and7(Ford'sfigs. 2 & 3, 1877), in the latter of which the animal, though not adult, has had the thorax and pygidium added to the head. The modifications in the size of the pleura are the chief changes. The anterior pair is reduced and retired within the posterior border of the head forming a semilunar arch joined with the occipital ridge in an angular bend. The posterior pair is enlarged and its spine is by and by reduced (figs.8,9,10) till it quite disappears and only the wide semicircular field between the first pleuron and the glabella is left behind as a remnant of its dilated body. It is to be remarked, that while in the plurality of the adult Olenellidæ all traces of the spiny projections of the second pleuron have vanished, they are still retained in the adult Holmia Kjerulfi, though not in the American Holmiæ, and thus give it at the same time a more ancient and a more larval stamp. The shallow groove along the back of its first pleuron indicates stronglyits pleural nature, as the thoracic pleura commonly are divided through such grooves. The same peculiarity is also observable in several of the American Olenellidæ. It is much the same with the posterior pleuron, the pleural nature of which is revealed through its spine, that is homologous and identical with the spiny terminations of the thoracic pleura. We have thus through the remarkable finds ofFordandWalcottcombined received an explanation of the morphological origin and nature of the facial ridge, the so called eye lobe and found that it has nothing whatever of the character of a visual organ. But it must be borne in mind, that these developmental changes are peculiar only to the Olenellidæ, the origin of the facial ridge in the later trilobites is, as we shall see, quite a different one.

The Olenellidæ belong chiefly to the oldest of the Cambrian beds with trilobites, and none of them has as yet been found higher up.

Next in order of evolution we have the important tribe of the Paradoxidæ. These are preeminently distinguished from the Olenellidæ through the well developed facial suture, which without exception in them all runs outside the facial ridge and separates this from the free cheek.[12]This is a great step forwards in the evolution and establishes the fact, demonstrable also by other evidence, that the formation of the facial suture is subsequent to the appearance of the facial ridge. This preexisting ridge seems to have had no small influence on the development of the suture, it checked its progress from the front or from the sides toward the fixed cheeks and directed its course against the genal angles. It lay as a protection for the glabella against this disseverance, causing the separating line to run along its outside.

[12]They have thus a quinquepartite cephalic shield, as the later trilobites.

This group consists of the genus Paradoxides proper, as well as of Centropleura, Metadoxides and Hydrocephalus, if this is an adult form and not the larva of an unknown Paradoxides. Perhaps such forms as »Conocephalites» EmmrichiBarrande, as well as Anomocare limbatum, An. excavatum, Bathyuriscus and Dolichometopus may on account of the shape of their facial ridges be considered as related to the Paradoxidæ. But this must be left for coming researches to decide. Some American Cambrian forms also share in this characteristic and may upon closer inspection be ranged here. So Zacanthoides. In these as in the true Paradoxidæ the facial suture follows the ridge along its whole length, while in the trilobites of the third group the facial suture is in contact only with the posterior end of the ridge, the so called eye lobe. Remopleurides does not show characters, that asBeecherthinks, could unite it with the Paradoxidæ. These are blind and Remopleurides has well developed eyes and an organization that gives it an isolated position in the system.

The facial ridge continues in a great variety of shapes, short or long, but always forming the fraction of a circle, always of nearly equal thickness, only slightly tapering towards one of the extremities, and always when in direct connection with the glabella, starting from the base of its foremost, largest segment. As a rule the ridge is more developed in the young or larval individuals, continuing from the glabella to near theposterior cephalic border in an uninterrupted arch[13]quite as in several of the adult Olenellidæ of the oldest Cambrian. It can be taken as granted that its origin is the same as in the Olenellidæ though at present the only evidence at hand is the small larva of Paradoxides oelandicus, whichLinnarssoncalled Parad. aculeatus.[14]In this we see the anterior pleuron or the facial ridge alone present, elongated downwards like the same pleuron in the figure 6 of Olenellus asaphoides and terminating like this in a fine spine stretching backwards outside the posterior border. Of the second pair of pleura there is nothing to be seen. This must then have been aborted at an earlier stage than in the Olenellidæ.

[13]G. F. Matthewhad before me, as I now find, pointed out this distinction in his memoir »Illustrations of the Fauna of St. John» N:o IV, p. 163. When he speaks of »the embryonic stage» in this and other passages he evidently means »larval stages», as the embryonic stages of necessity must remain unknown to us. (Later remark.)[14]Om faunan i lagren med Paradoxides ölandicus (1877), p. 359, pl. 14, f. 11.

[14]Om faunan i lagren med Paradoxides ölandicus (1877), p. 359, pl. 14, f. 11.

The connexion between the free cheeks and the middle part of the head has been very lax not only in the Paradoxidæ, but on the whole in nearly all Cambrian trilobites with free cheeks. When the free cheek is dissevered it shows no trace of the ridge, there is only a large scallop on the spot where it embraced the ridge. In the Paradoxidæ the rim of the indenture and the ridge are in so close contact that there is not the least place for an eye between them, as can be seen in the few specimens with a complete head. In all oculate trilobites again without any exception the facial suture separates that part of the eye which is the real visual organ with corneal facets, from the interior often elevated portion, opposite it, the so called palpebral lobe. The eye isalwaysplaced on the free cheek,[15]the lobe again always on the fixed cheek of the head shield. No real eye exists without the palpebral lobe, and, on the other hand, that part of the facial ridge which later in the development changes to a palpebral lobe, occurs alone without any eye in a great number of Cambrian trilobites, and consequently these are blind and such is the case with the Paradoxidæ and a great number of the succeeding.

[15]Excepting in Harpes, which has no free cheeks.

There is not the least evidence to support the suggestion that the »ocular ridge» is homologous, with the eye of Apus[16]and that the real crystal cones lay sunk beneath the surface in a »water sac». As we, for instance, in Peltura have an »ocular ridge» (= facial ridgemihi) on the fixed cheek and opposite its posterior extremity, the »eye lobe», a real eye with facets on the free cheek it is not likely to suppose that the »ocular ridge» nor the »eye lobe» ever functioned as a visual organ or that two widely different sorts of eyes were placed in closest vicinity opposite each other.

[16]Bernard, The systematic position of the Trilobites. Qu. Journ. Geol. Soc. 1894, p. 411.

It may be worth while here to remind of the great similarities, whether analogous or homologous, in the formation of the superior surface of the head in the trilobites and the embryons and the newly hatched larva of Limulus. The latter have the head shield separated into five portions, partly through a facial suture which, as in Peltura, Dalmanites and others, forms a continuous line around the glabella. This suture divides the eye node in a similar way as in the trilobites, that is, sectioning it in two parts, of whichone adheres to the central fixed cheeks and the other to the free cheeks.[17]And to judge by the figures ofKingsleythe former, the whitish moiety is the first developed and sometimes for a while quite alone as the facial ridge of the trilobites and probably also anterior to the suture, as this is not complete at this stage. This white node reminds of the small facial ridge in Arionellus ceticephalusBarr.

[17]Packard, Development of Limulus polyphemus. Memoirs Boston Soc. Nat. H. vol. II, pl. V, fig. 25. Nothing is said about the exterior structure of the eyes.Dohrn, Zur Embryologie and Morphologie des Limulus Polyphemus. Jenaische Zeitschrift 1871. A very good figure (pl. XIV, f. 4) shows clearly the two parts of the eyes, the interior one being larger.Kingsleyalso (Devel. of Limulus, Journal of Morphology, vol. 7, 1892, pl. VI, fig. 34) has in the last larval stage the suture and the eye in two parts, of which one is white lying inside the suture and the eye proper, black, outside.

[17]Packard, Development of Limulus polyphemus. Memoirs Boston Soc. Nat. H. vol. II, pl. V, fig. 25. Nothing is said about the exterior structure of the eyes.

Dohrn, Zur Embryologie and Morphologie des Limulus Polyphemus. Jenaische Zeitschrift 1871. A very good figure (pl. XIV, f. 4) shows clearly the two parts of the eyes, the interior one being larger.

Kingsleyalso (Devel. of Limulus, Journal of Morphology, vol. 7, 1892, pl. VI, fig. 34) has in the last larval stage the suture and the eye in two parts, of which one is white lying inside the suture and the eye proper, black, outside.

AsKishinouya[18]has already pointed out and as I have anticipated above the head of the most developed trilobites in their adult state, and the head of the larval Limulus consists of five parts, viz. 1) the glabella in the centre, 2 & 3) the fixed cheeks, 4 & 5) the free cheeks. An elevated ridge in the adult Limulus shows where the suture once lay and it is on the outside thereof that the eye of the adult is placed. What other authors call the ridge or the eye ridge in Limulus,Kishinouyarightly names a suture.

[18]Journal College of Science Tokio, vol. V, p. 53, 1892.

It was the renowned Swedish naturalistWahlenberg, who first recognized the importance of the facial suture, which he called »linea ocularis»,[19]but to another Swedish palæontologistDalman[20]the exact definition of this suture is due, to which he gave the name still in use. He expressly remarks that the suture crosses the ocular node and limits the outside of the »lobus palpebralis» and he makes a clear distinction between that lobe and the »tuberculi and eminentiæ oculares» (= facial ridge) of which he says that they are the more or less evident elevations situated in theblindPalæades on the place of the eyes (in which he is wrong) »and which perhaps are an indication of such organs». But then he says doubtfully (p. 255) »tuberculorum ocularium veram naturam determinare haud ausi sumus, etsi oculorum formam sat bene exhibere videantur» and he adds concerning Paradoxides »oculi nulli, eorum loco autem tuberculi duo».

[19]Additamenta quædam ad petrificata telluris Suecana, in Acta Upsaliensia, vol VIII, 1821, p. 294.[20]Vet.-Akad. Handl. 1826 p. 126.

[19]Additamenta quædam ad petrificata telluris Suecana, in Acta Upsaliensia, vol VIII, 1821, p. 294.

[20]Vet.-Akad. Handl. 1826 p. 126.

Group 2. The Olenidæ and related families.

Next in the ascending order we have the largest group of trilobites in the Cambrian, of which the greatest part is formed by the Olenidæ. A facial ridge different in shape and different in development from that in the former group characterizes them. Unlike the semilunate ridge of the Olenellidæ it issues mostly from the front of the first segment of the glabella and goes generally backwards till it meets the facial suture. It is narrow and fine as a thread, but for the rest assumes a great variety of forms. It may be curved as a circle segment as in Sao, Liostracus, it may be long and straight, standing out in a right angle to the glabella as in Eurycare, it may be short and straight in an acute angle to the glabella as in Parabolina and so forth. In the same genus, asfor instance Olenus, straight and curved ridges occur in the various species, and consequently the form of this sort of ridge cannot be used as a generic character. It moreover differs from the ridge in the Olenellidæ and the Paradoxidæ in having the posterior extremity widened as a tubercle or node, which commonly has been called the »palpebral lobe», while in the older groups the ridge is only at the most a little pointed in both ends or of equal thickness.

Some persistent archaic genera as Conocoryphe and Elyx inform us how this ridge has originated and how widely different it is from that in the Olenellidæ. On the surface of their head shield no ridge is visible, but on the interior side of it we behold, as the figures show (Pl. VI, figs. 43 & 44) on each side of the glabella a ramifying system as of the most minute vessels, which spread and cover the whole surface of the cheeks. This reticulation issues from a main trunk that goes nearly straight in an oblique direction from the foremost segment of the glabella and emits narrow branches forwards and backwards on its both sides. And these branches go on dividing till they occupy the entire interior surface of the fixed cheeks. The figures representing this are taken from Elyx laticepsfig. 43, and an almost similar image has been obtained from a species of Conocoryphefig. 44.

Now the question may be asked, what does this network of branching canals mean? I think we cannot gain a more plausible answer than that given us through the inspection of the soft parts that lie hidden behind the glabella of Limulus and its fixed cheeks, the only living crustacean which offers the greatest homologies with the head shield of the trilobites. Next below the shell of the glabella and the cheeks of Limulus there is a complicated stratum of muscles and behind this the heart and the great central circulatory system spreading from that centrum towards the sides of the head shield, the vessels being the more fine and minute, the more they are elongated from the centre. I now suppose that likewise in these trilobites the narrow and prominent glabella has been the receptacle not only of a strong mass of muscles, but also for the central part of the circulation. This centre has there sent out two relatively strong ducts or canals, one on each side of the glabella feeding all soft tissues inside and near the cheeks and probably also other important parts of the body. It is also clearly seen in Elyx how the orifice of the main trunc opens in the hollow of the glabellar apex.

In all crustaceans, as far as is now known, their more or less hard calcareous or chitinous skeleton is moulded by the subjacent tissues and glands. The sculpture of the surface consequently is an outcome of the fashioning procedures showing what has been going on below and what is still going on. If we then on the exterior surface of the head shield of the trilobites see the radiating lines in relief we must conclude that they are due to a subjacent system of almost capillary vessels causing hollow impressions on the inside and elevated ridges exteriorly. In Elyx the vessels have made no strong impression as to be visible on the outside. In a couple of American Conocoryphæ again (C. reticulata and C. trilineata) the main trunc of the vessels has formed a short faintly elevated ridge. In Solenopleura (Pl. VI, f. 45) the facial ridge has been fully developed, and by the casts of the inside it is clearly seen that the main trunc of the vessels makes the inside of the ridge and has been much incrassated. On the other hand the smaller branches issuingfrom it have been reduced in size and number, but are still visible. Thus, if I am right, the two, the facial ridge and the main trunc, are in causal connection and the former has been moulded by the later, when it had gained in size sufficiently, and when a richer affluxion of nutritive fluid was directed backwards towards the point where the eye at last originated. The further changes in this ridge thus are related to the development of the eye. As far as I am aware, there are as yet no data to tell us whether the appendages or pleura of the glabella in the larval (or primordial) Olenellidæ have the same origin as the ridge of the Olenidæ or are homologous with it. Perhaps it may be so in respect to the foremost one, which also is persisting. It is, however, not known how the pleura in the olenellid glabella have been developed, whereas it is well ascertained by the growth of the glabella in Sao, Liostracus and others that such pleura never have been developed in them.

What the phylogenetic evolution has taught us concerning the formation of the facial ridge is confirmed by the ontogenetic development of individuals of some species as for instance Sao hirsutaBarr., Liostracus sp.Brög.and others.

This development can be followed in detail through the excellent figures of the larval stages of Sao hirsuta whichBarrandehas given in his magnificent work, vol. I, pl. 7. In thefirststage figured (figs. 1 d-e) the whole animal consists only of the head shield which is completely smooth, the glabella scarcely segmented, no facial ridge, no facial suture. In thesecondstage, according to me (figs. 2-4 a), the glabella has become distinctly segmented, and the pygidium and partly the thorax have been added. In thethirdstage, 4 c, d-9, one small narrow string exits from each side of the front of the glabella, making the first faint beginning of the facial ridge. They form a right angle with the glabella. In continued growth they become by degrees a little more curved (figs. 5 c, 6 b) and the lengthened ridge bends parallel to the outer lateral margin of the head (fig. 9 b). In fig. 9 d it has become so far complete that it reaches nearly back to the posterior cephalic margin, but is still of the same narrowness all along. In thefourthstage, in a specimen (fig. 10 a) of 3 mm. in length, the facial suture makes its first appearance, setting in from the posterior margin of the head and meeting in its forward progress the posterior extremity of the facial ridge which now begins to swell out. It seems that both have a mutual influence on each other, the suture being deviated from a straight course[21]to take a bend outside the ridge, and the extremity of the ridge again at this contact to increase in size so as to form the elongated tubercle, often called palpebral lobe (fig. 14 b). This now augments in the same rate as the whole body. The characters of the four stages of the development of the larva consequently are: 1:o The archaic stage, only head shield with ridgelike glabella. 2:o The coming and growth of the pygidium and the complete segmentation of the glabella. 3:o The coming and growth of the facial ridge. 4:o The coming and development of the facial suture.

Back to Index Next