FOOTNOTES:

FOOTNOTES:[151]Bulletin Geol. Soc. America, 1892.

[151]Bulletin Geol. Soc. America, 1892.

Ostracophores.—Among the earliest vertebrates actually recognized as fossils belongs the group known asOstracophori(ὄστρακος, a box; φορέω, to bear). These are most extraordinary creatures, jawless, apparently limbless, and enveloped in most cases anteriorly in a coat of mail. In typical forms the head is very broad, bony, and horseshoe-shaped, attached to a slender body, often scaly, with small fins and ending in a heterocercal tail. What the mouth was like can only be guessed, but no trace of jaws has yet been found in connection with it. The most remarkable distinctive character is found in the absence of jaws and limbs in connection with the bony armature. The latter is, however, sometimes obsolete. The back-bone, as usual in primitive fishes, is developed as a persistent notochord imperfectly segmented. The entire absence of jaw structures, as well as the character of the armature, at once separates them widely from the mailedArthrodiresof a later period. But it is by no means certain that these structures were not represented by soft cartilage, of which no traces have been preserved in the specimens known.

Nature of the Ostracophores.—The Ostracophores are found in the Ordovician or Lower Silurian rocks, in the Upper Silurian, and in the Devonian. After the latter period they disappear. The species are very numerous and varied. Their real affinities have been much disputed. Zittel leaves them with the Ganoids, where Agassiz early placed them, but they show little homology in structure with the true Ganoids. Some have regarded them as aberrant Teleosts, possibly as freakish catfishes. Cope saw in them a huge mailed group of archaic Tunicates, while Patten has soberly and with considerable plausibility urged their affinity[153]to the group of spiders, especially to the horseshoe-crabs (Limulus) and their palæozoic ancestors, theEurypteridæandMerostomata.

The best guess as to the affinities of the Ostracophores is perhaps that given by Dr. Ramsey H. Traquair ("Fossil Fishes of the Silurian Rocks of the South of Scotland," 1899). Traquair regards them as highly aberrant sharks, or, more exactly, as being derived, like the Chimæras, from a primitive Elasmobranchstock. In favor of this view is the character of their armature, the bony plates themselves to be regarded as formed by the fusion of shagreen grains or scales. According to Traquair: "Specialization from the most specialized form,Lanarkia, has been accompanied by (1) fusion of the spinelets (Lanarkia) or shagreen grains (Thelodus) into plates, scutes, and rhombic scales, supported by hard matter developed in a deeper layer of skin, and (2) alterations in the pectoral fin-flaps, which, becoming covered up by the postero-lateral plates inDrepanaspis, are finally no longer recognizable in thePteraspidæ."

Fig. 353.—Odontotodus schrencki(Pander) (Tremataspis), ventral side. Island of Oesel. (After Patten.)

Fig. 354.—Odontotodus schrencki(Pander) (Tremataspis), dorsal side. Island of Oesel. (After Patten.)

Woodward leaves their exact relationship undefined, while others have regarded them as mailed lampreys, at any rate to be excluded from theGnathostomi, or jaw-bearing series. The apparent absence of true jaws, true limbs, and limb-girdles certainly seems to separate them widely from true fishes, but these characters are negative only, perhaps due to degeneration, and at any rate they are not yet absolutely determined. Certainly they offer no positive proof of affinity with the modern Cyclostomes.

Dr. Traquair regards theHeterostracior most primitiveOstracophoresas most certainly derived from the Elasmobranchs. Other writers have attacked the integrity of the group of Ostracophores, questioning the mutual relationship of its component parts. Reiss, for example, regards the association of theOsteostraciwith theHeterostracias "unbegründet" and "unheilvoll," while Ray Lankester, as quoted by Traquair, affirms that "there is absolutely no reason for regardingCephalaspisas allied toPteraspisbeyond that the two genera occur in the same rocks, and still less for concluding that either has any connection withPterichthys." Elsewhere Lankester states that theHeterostraciare associated at present with theOsteostraci, "because they have, likeCephalaspis, a large head-shield, and because there is nothing else with which to associate them." Patten, on the other hand, seems inclined to deny the rank ofHeterostraciandOsteostracias even separate orders, regarding them as very closely related to each other as also to their supposed spider-like ancestors.

Fig. 355.—Head ofOdontotodus schrenckiPander, from the side. (After Patten.)

But the consensus of opinion favors the belief that the four orders usually included under this head are distinct and at the same time are really related one to another. For our purposes, then, we may regard theOstracophorias a distinct class of vertebrates. By placing it after the Elasmobranchs we may indicate its probable descent from a primitive shark-like stock.

Fig. 356.—The Horseshoe Crab or King-crab,Limulus polyphemusLinnæus. Supposed by Professor Patten to be an ally of the Ostracophores; usually regarded as related to the Spiders.

On this subject Dr. Dean remarks: "The entire problem of the homology of the dermal plates and 'scales' in the Ostracophores and Arthrognaths is to the writer by no means as clear as previous writers have conceded. From the histological standpoint, admitting the craniote nature of the vasodentine and cancellous layers in the dermal plates, it nevertheless does not follow that they have been derived from the actual conditions of the dermal denticles of the ancestral Gnathostome, as were unquestionably the dermal plates of Teleostomes and Dipnoans. It seems equally if not more probable, on the other hand, that the dermal armoring of the distinct groups may have had an altogether different mode of origin, the product of a crude evolution which aimed to strengthen the skin by a general deposition of calcareous matter throughout its entire thickness. The tuberculation of plates thus acquired might have become an important step in the development of a more superficial type of armoring which is most preferably represented by the dermal denticles of Selachians. Nor, in passing, need the presence of a mucus-canal system in the early plated forms be of greater morphological importance than a foreshadowing of the conditions of Gnathostomes, for this system of organsmight serve as well as evidence, in a general way, of relationship with Marsipobranchs. Nor is this evidence the more conclusive when we reflect thatno known type of Gnathostome, recent or fossil, possesses open sensory grooves in distinct dermal plates. The presence, furthermore, of a dorsal fin and a 'truly piscine heterocercal tail,' as noted by Traquair, is by no means as Gnathostome-like as these structures at first glimpse appear. For they lack not merely the characteristic radial supports of fishes, but even actinotrichia. Their mode of support, on the other hand, as Smith Woodward points out, is of a more generalized nature, bent scales, homologous with those of the adjacent body region, taking the place of the piscine external supports." The actual position in the system to be finally assigned to the Ostracophores is therefore still uncertain.

Orders of Ostracophores.—Four orders ofOstracophoriare now usually recognized, known in the systems of Woodward and Traquair asHeterostraci,Osteostraci,Antiarcha, andAnaspida. The former is the most primitive and perhaps the most nearly allied to the sharks, the second is not very remote from it, the last two aberrant in very different directions. Hay places theAntiarchawith theArthrodiraunder the superorder ofPlacodermi.

Order Heterostraci.—TheHeterostraci(ἕτερος], different; ὀστράκος, box) have no bone-corpuscles in the coat of mail. This typically consists of a few pieces above, firmly united and traversed by dermal sense-organs or "lateral lines." The ventral shield is simple. Four families are recognized by Traquair as constituting theHeterostraci, these forming a continuous series from shark-like forms to the carapace-coveredPteraspis. In the most primitive family, theThelodontidæ,[154]the head and trunk are covered with small scales or tubercles of dentine and not fused into large plates. The tail is slender and heterocercal, the caudal fin deeply forked. Until lately these tubercles were regarded as belonging to sharks, and they are still regarded by Traquair as evidence of the affinity of theHeterostraciwith theAcanthodei. Dr. Traquair thinks that a flap or lappet-like projection behind the head may bea pectoral fin. The three known genera areThelodus,Lanarkia, andAteleaspis. InThelodusthe scales consist of a base and a crown separated by a constriction or neck.Thelodus scoticus,Thelodus pagei, andThelodus planusare found in the Silurian rocks of Scotland. Other species, asThelodus tulensisof Russia, extend to the Upper Devonian.

InLanarkiathe large sharp scales have an expanded base like the mouth of a trumpet.Lanarkia horridaandL. spinulosaare found in the shire of Lanark in Scotland. InAteleaspis(tesselatus) the skin is covered with small polygonal plates. The lateral flaps or possibly fins take the form of flat rhombic sculptured scales. In this genus the eyes seem to be on the top of the head.

Fig. 357.—Lanarkia spinosaTraquair. Upper Silurian. FamilyThelodontidæ. (After Traquair.)

In thePsammosteidæof the Devonian the head is covered with large plates which are not penetrated by the sense-organs. These plates are covered with minute, close-set tubercles, covered with brilliant ganoid enamel and with finely crimped edges. According to Dr. Traquair, these tubercles are shagreen granules which have coalesced and become united to plates formed in a deeper layer of the skin, as inAteleaspisthe minute scales have run together into polygonal plates. These creatures have been considered as "armored sharks," and Dr. Traquair regards them as really related to the acanthodean sharks. Nevertheless they are not really sharks at all, and they find their place with thePteraspisand other longer known Heterostracans.

The family ofDrepanaspidæconsists of a single recently known species,Drepanaspis gmundenensis, found in a pyritized conditionin purple roofing-slate in Gmünden, Germany. This fish, which reaches a length of about two feet, has a broad head, with eyes on its outer margin, with a slender body and heterocercal tail. The head has a broad median plate and smaller polygonal ones. The flaps, supposed to represent the pectoral fins, are here cased in immovable bone. No trace of internal skeleton is found by Traquair, who has given the restoration of this species, but the mouth has been outlined.

Fig. 358.—Drepanaspis gmundenensisSchlüter. Upper Silurian, Gmünden, Germany. (After Traquair.)

The best known of the Heterostracan families is that ofPteraspidæ. In this family the plates of the head are coalesced in a large carpace, the upper part originally formed of seven coalesced pieces. A stout dorsal spine fits into a notch of the carapace. The slender body is covered with small scales and ends in a heterocercal tail. The dermal sense-organs are well developed.Pteraspis rostrataoccurs in the Lower Devonian. Other genera arePalæaspisandCyrthaspis.

Fig. 359.—Pteraspis rostrataAgassiz. Devonian. FamilyPteraspidæ. (After Nicholson.)

Order Osteostraci.—The Osteostraci (ὄστεον, bone; οστρακος, box) (calledAspidocephaliby Rohon) have bone-corpuscles in the shields, and the shield of the back is in one piece withoutlateral-line channels or sense-organs. Ventral shield single. The order includes three families. TheCephalaspidæhave the shields tuberculate, the one between the eyes fixed, and the anterior body-shields are not fused into a continuous plate. The best known of the numerous species isCephalaspis lyellifrom the Lower Devonian of England.Hemicyclaspis murchisonioccurs in the Upper Silurian of England, and the extraordinaryCephalaspis dawsoniin the Lower Devonian of Gaspé, Canada.Eukeraspis pustuliferahas the head-shield very slender and armed with prickles. In theThyestidæthe anterior body-scales are fused into a continuous plate.ThyestisandDidymaspisare genera of this type. TheOdontotodontidæ(Tremataspidæ) have the shield truncate behind, its surface finely punctate, and the piece between the eyes not fixed.Odontotodus[155]schrenkiis found in the Upper Silurian of the Island of Oesel in company with species ofThyestes. TheEuphaneropidæare represented in the Devonian of Quebec.

Fig. 360.—Cephalaspis lyelliAgassiz, restored. (After Agassiz.)

Order Antiarcha.—The Antiarcha (ἀντί, opposite; ἀρχός, anus) have also bone-corpuscles in the plates, which are also enameled. The sense-organs occupy open grooves, and the dorsal and ventral shields are of many pieces. The head is jointed on the trunk, and jointed to the head are paddle-like appendages, covered with bony plates and resembling limbs. There is no evidence that these erectile plates are real limbs. They seem to be rather jointed appendages of the head-plate, erectile on a hinge like a pectoral spine. There are traces of ear-cavities, gill-arches, and other fish-like structures, but nothing suggestive of mouth or limbs.

This group contains one family, theAsterolepidæ, with numerous species, mostly from Devonian rocks. The best known genus isPterichthyodes,[156]in which the anterior median plateof the back is overlapped by the posterior dorso-lateral.Pterichthyodes millerifrom the Lower Devonian, named by Agassiz for Hugh Miller, is the best known species, although numerous others, mostly from Scottish quarries, are in the British Museum.Asterolepis maximusis a very large species from the same region, known from a single plate.Bothriolepis canadensisis from the Upper Devonian of Scaumenac Bay near Quebec, numerous specimens and fragments finely preserved having been found.

Fig. 361.—Cephalaspis dawsoniLankester. Lower Devonian of Canada. FamilyCephalaspidæ. (After Woodward.) In the square a portion of the tubercular surface is shown.

Microbrachium dickiwith the pectoral appendages small occurs in the Devonian of Scotland.

The earliest remains ofOstracophoriare found in Ordovician or Lower Silurian rocks of the Trenton horizon at CañonCity, Colorado. These consist of enormous numbers of small fragments of bones mixed with sand. With these is a portion of the head carapace of a small Ostracophore which has been named by Dr. WalcottAsteraspis desiderataand referred provisionally to the family ofAsterolepidæ, which belongs otherwise to the Lower Devonian.

Fig. 362.—Pterichthyodes testudinarius(Agassiz), restored. Lower Devonian FamilyAsterolepidæ. (After Traquair and others.)

With these remains are found also scales possibly belonging to a Crossopterygian fish (Eriptychius). These remains make it evident that the beginning of the fish series lies far earlier than the rocks called Silurian, although fishes in numbers are not elsewhere known from rocks earlier than the Ludlow shales of the Upper Silurian, corresponding nearly to the Niagara period in America.

In the Ludlow shales we find the next appearance of theOstracophores, two families,ThelodontidæandBirkeniidæ, being there represented.

Fig. 363.—Pterichthyodes testudinariusAgassiz, side view. (After Zittel, etc.)

Fig. 364.—Birkenia elegansTraquair. Upper Silurian. (After Traquair.)

Order Anaspida.—Recently a fourth order,Anaspida(ἄ, without; ἀσπίς, shield), has been added to theOstracophorithrough the researches of Dr. Traquair. This group occurs in the Upper Silurian in the south of Scotland. It includes the single familyBirkeniidæ, characterized by the fusiform body, bluntly rounded head, bilobate, heterocercal tail, and a median row of hooked spinous scales along the ventral margin. No trace of jaws, teeth, limbs, or internal skeleton has been found. Unlike other Ostracophores,Birkeniahas no cranial buckler with orbits on the top, nor have the scales and tubercles the microscopic structure found in other Ostracophores. In the genusBirkeniathe head and body are completely covered by tubercular scutes. The gill-openings seem to be represented by a series of small perforations on the sides. A dorsal fin is present.Birkenia elegansis from the Ludlow and Downstonian rocks of southern Scotland.Lasianius problematicusfrom the same rocks is very similar, but is scaleless. It has a row of ventral plates like those ofBirkenia, the only other hard parts itpossesses being a number of parallel rods behind the head, homologous with the lateral series ofBirkenia.Lasianiusis therefore a specialized and degenerate representation ofBirkenia, differing somewhat as "the nearly nakedPhanerosteondiffers from otherPalæoniscidæwhose bodies are covered with osseous scales."

Fig. 365.—Lasianius problematicusTraquair. Upper Silurian. (After Traquair.)

FOOTNOTES:[152]This group was first called by CopeOstracodermi—a name preoccupied for the group of bony trunkfishes,Ostracidæ. The still earlier name ofPlacodermi, chosen by McCoy (1848), was intended to include Arthrodires as well as Ostracophores. Rohon (1892) calls the groupProtocephali, and to the two orders he assigns the namesAspidorhiniandAspidocephali. These groups correspond toHeterostraciandOsteostraciof Woodward. Another name of early date is that ofAspidoganoidei, given by Professor Gill in 1876, but not defined until 1896. These fishes are, however, not "Ganoids" and the nameOstracophoriseems to receive general preference. The groupPeltacephalataof Patten corresponds essentially toOstracophori, as does also the orderHypostomataof Gadow.[153]According to Professor Patten's view, the close resemblance of the shields ofPteraspisto those of contemporaneousEurypteridsindicates real affinity. But theEurypteridsare related to the spiders and toLimulus. The only reason for thinking thatPteraspisis a fish at all lies in its resemblance toCephalaspis, which is in several ways fish-like, although its head shield is much like that ofLimulus. All these resemblances in Patten's view indicate real affinity. Patten considers thePteraspidsas derived from primitive arachnid or spider-like forms having a bony carapace asLimulushas. FromPteraspishe derives the other Ostracophores, and from these the sharks and other vertebrates, all of which appear later in time than the earliest Ostracophores. This view of the origin of vertebrates is recently urged with much force by Professor Patten (Amer. Nat., 1904, 1827). Most naturalists regard such resemblances in specialized structures on the outside of an animal as parallelisms due to likeness in conditions of life. The external structure in forms of really different nature is often similarly modified. Thus certain catfishes, pipefishes, sea-moths, and agonoid fishes are all provided with bony plates not unlike those of ganoid fishes, although indicative of no real affinity with them. Commonly the ancestry of vertebrates is traced through enteropneustans to soft-bodied worms which have left no trace in the rocks.In the same connection, Professor Patten suggests that the lateral fold from which many writers have supposed that the limbs or paired fins of vertebrates is evolved is itself a resultant of the fusion of the fringing appendages on the sides of the body. Such appendages are found in the primitive mailed arachnoids and inLimulus. They are shown very plainly in Patten's restoration ofCephalaspis. About thirty of them of a bony nature and jointed to the body occur on either side between the gill opening and the vent.[154]CalledCœlolepidæby Pander and Traquair, butCœlolepisis a later synonym ofThelodus.[155]This name, inappropriate or meaningless, is older thanTremataspis.[156]The earlier name ofPterichthyshas been already used for a genus of living fishes.

[152]This group was first called by CopeOstracodermi—a name preoccupied for the group of bony trunkfishes,Ostracidæ. The still earlier name ofPlacodermi, chosen by McCoy (1848), was intended to include Arthrodires as well as Ostracophores. Rohon (1892) calls the groupProtocephali, and to the two orders he assigns the namesAspidorhiniandAspidocephali. These groups correspond toHeterostraciandOsteostraciof Woodward. Another name of early date is that ofAspidoganoidei, given by Professor Gill in 1876, but not defined until 1896. These fishes are, however, not "Ganoids" and the nameOstracophoriseems to receive general preference. The groupPeltacephalataof Patten corresponds essentially toOstracophori, as does also the orderHypostomataof Gadow.

[153]According to Professor Patten's view, the close resemblance of the shields ofPteraspisto those of contemporaneousEurypteridsindicates real affinity. But theEurypteridsare related to the spiders and toLimulus. The only reason for thinking thatPteraspisis a fish at all lies in its resemblance toCephalaspis, which is in several ways fish-like, although its head shield is much like that ofLimulus. All these resemblances in Patten's view indicate real affinity. Patten considers thePteraspidsas derived from primitive arachnid or spider-like forms having a bony carapace asLimulushas. FromPteraspishe derives the other Ostracophores, and from these the sharks and other vertebrates, all of which appear later in time than the earliest Ostracophores. This view of the origin of vertebrates is recently urged with much force by Professor Patten (Amer. Nat., 1904, 1827). Most naturalists regard such resemblances in specialized structures on the outside of an animal as parallelisms due to likeness in conditions of life. The external structure in forms of really different nature is often similarly modified. Thus certain catfishes, pipefishes, sea-moths, and agonoid fishes are all provided with bony plates not unlike those of ganoid fishes, although indicative of no real affinity with them. Commonly the ancestry of vertebrates is traced through enteropneustans to soft-bodied worms which have left no trace in the rocks.In the same connection, Professor Patten suggests that the lateral fold from which many writers have supposed that the limbs or paired fins of vertebrates is evolved is itself a resultant of the fusion of the fringing appendages on the sides of the body. Such appendages are found in the primitive mailed arachnoids and inLimulus. They are shown very plainly in Patten's restoration ofCephalaspis. About thirty of them of a bony nature and jointed to the body occur on either side between the gill opening and the vent.

In the same connection, Professor Patten suggests that the lateral fold from which many writers have supposed that the limbs or paired fins of vertebrates is evolved is itself a resultant of the fusion of the fringing appendages on the sides of the body. Such appendages are found in the primitive mailed arachnoids and inLimulus. They are shown very plainly in Patten's restoration ofCephalaspis. About thirty of them of a bony nature and jointed to the body occur on either side between the gill opening and the vent.

[154]CalledCœlolepidæby Pander and Traquair, butCœlolepisis a later synonym ofThelodus.

[155]This name, inappropriate or meaningless, is older thanTremataspis.

[156]The earlier name ofPterichthyshas been already used for a genus of living fishes.

TheArthrodires.—Another large group of extinct fishes mailed and helmeted is included under the general name ofArthrodira[157](ἄρθρος, joint; δεῖρα, neck), orArthrognathi(ἄρθρος, γνάθος, jaw), the latter term recently framed by Dr. Dean with a somewhat broader application than the former.

These fishes differ from the Ostracophores, on the one hand, in the possession of jaws and in the nature of their armored covering. On the other hand, the nature of these jaws, the lack of differentiation of the skeleton, and the uncertain character of the limbs separate them still more widely from the true fishes. Their place in the system is still unknown, but their origin seems as likely to be traceable to Ostracophores as to any other group.

The head in all the species is covered with a great bony helmet. Behind this on the nape is another large shield, andbetween the two is usually a huge joint which Dr. Dean compares to the hinge of a spring-beetle (Elater).

As to the presence of limbs, no trace of pectoral fin or anterior limb has been found. Dean denies the existence of any structures corresponding to either limb, but Woodward figures a supposititious posterior limb inCoccosteus, finding traces of basal bones which may belong to it.

These monstrous creatures have been considered by Woodward and others as mailed Dipnoans, but their singular jaws are quite unlike those of theDipneusti, and very remote from any structures seen in the ordinary fish. The turtle-like mandibles seem to be formed of dermal elements, in which there lies little homology to the jaws of a fish and not much more with the jaws of Dipnoan or shark.

The relations with the Ostracophores are certainly remote, though nothing else seems to be any nearer. They have no affinity with the true Ganoids, to which vaguely limited group many writers have attached them. Nor is there any sure foundation to the view adopted by Woodward, that they are to be considered as armored offshoots of the Dipnoans.

According to Dean we might as well refer the Arthrodires to the sharks as to the Dipnoans. Dean further observes ("Fishes Living and Fossil"):

Fig. 366.—Coccosteus cuspidatusAgassiz, restored. Lower Devonian. (After Traquair, per Woodward.)

"The puzzling characters of the Arthrodirans do not seem to be lessened by a more definite knowledge of their different forms. The tendency, as already noted, seems to be at present to regard the group provisionally as a widely modified offshoot of the primitive Dipnoans, basing this view upon their general structural characters, dermal plates, dentition, autostylism. But only in the latter regard could they have differed morewidely from the primitive Elasmobranch or Teleostome, if it be admitted that in the matter of dermal structures they may be clearly separated from the Chimæroid. It certainly is difficult to believe that the articulation of the head of Arthrodirans could have been evolved after dermal bones had come to be formed, or that a Dipnoan could become so metamorphosed as to lose not only its body armoring, but its pectoral appendages as well. The size of the pectoral girdle is, of course, little proof that an anterior pair of fins must have existed, since this may well have been evolved in relation to the muscular supports of plastron, carapace, trunk, and head. The intermovement of the dental plates, seen especially inDinichthys, is a further difficulty in accepting their direct descent from the Dipnoans."

Fig. 367.—Jaws ofDinichthys hertzeriNewberry. Upper Devonian. Ohio. (After Newberry.)

Occurrence of Arthrodires.—These fishes occur in abundance from the Silurian times to the Mesozoic. In the Devonian their gigantic size and thick armor gave them the leading position among the hosts of the sea. Among the genera there occurred "series of forms most interesting as to their evolution." "It is found more and more evident," says Dr. Dean ("Fishes, Living and Fossil," pp. 135, 136) "that the Arthrodirans may have represented the dominant group in the Devonian period, as were the sharks in the Carboniferous, or as are the Teleosts in modern times. There were forms which, likeCoccosteus, had eyes at the notches of the head-buckler; others, likeMacropetalichthys, in which orbits were well centralized; some, likeDinichthysandTitanichthys, with the pineal foramen present; some with pectoral spines(?); some with elaborately sculptured dermal plates. Among their forms appear to have been those whose shape was apparently subcylindrical, adapted for swift swimming; others (Mylostoma) whose trunk was depressed to almost ray-like proportions. In size they varied from that of the perch to that of a basking shark. In dentition they presented the widest range in variation, from the formidable shear-like jaws ofDinichthysto the lip-like mandibles ofTitanichthys, the tearing teeth ofTrachosteus, the wonderfully forked tooth-bearing jaw-tips ofDiplognathus, to the Cestraciont type,Mylostoma. The latter form has hitherto been known only from its dentition, but now proves to be, as Newberry and Smith Woodward suggested, a typical Arthrodiran."

Classification of Arthrodira.—Our knowledge of the systematic relations of the Arthrodira is mostly of recent origin. Woodward refers most of the remains to the best known genusCoccosteus, and recognizes as families theCoccosteidæ,Mylostomidæ,Asterosteidæ, andPhyllolepidæ.

Fig. 368.—An Arthrodire,Dinichthys intermediusNewberry, restored. Devonian, Ohio. (After Dean.)

Dr. Bashford Dean in different papers has treated these fishes in great detail. In a recent paper on the "Relationships of theArthrognathi"[158]he recognizes the group as a class coordinate withCyclostomiandElasmobranchii. This class, which he callsArthrognathi, is first divided into two suborders,Anarthrodira, without joint at the neck, andArthrodira, with such a joint. The former comprises one order,Stegothalami, and the latter two orders,TemnothoraciandArthrothoraci. The following is Dr. Dean's definition of these orders and their component families:

Arthrognathi.—"Chordates whose anterior body region is encased in dermal elements, and divisible by a more or less definite partition into head and trunk. Dermal plates which surround the mouth function as jaws. No evidence of branchial arches. Column notochordal, showing no traces of centra; well-marked neural and hæmal elements. Paired limbs [absent or uncertain]. Dermal plates consisting typically of two layers, the superficial tuberculate, the inner bony with radiating lamellæ.Orbits situated near or at the margin of the head-shield and separated from one another by fixed integumental plates. A pineal funnel present situated in a fixed plate. A mucous system whose canals radiate from the preoccipital region."

Anarthrodira.—"Arthrognaths in which the cranial and dorsal regions are separated by a fixed partition whose dorsal rim is overlapped and concealed by superficial plates. Of these a large median dorsal element is present which extends backward superficially from the region near the pineal funnel. Also a pair of elements which overlie the position of the external occipital joint. Suborbital plates apparently absent. Jaw elements undescribed."

Stegothalami(στέγος, roof; θάλαμος, chamber).—"Anarthrodires in which the cranio-dorsal septum is vertical and deep, its height equal apparently to that of the arch of the head-shield. By this deep partition the latter appears to inclose two chambers (whence the ordinal name). Orbits inclosed by pre- and postorbital plates. Mucous system lacks a postorbital canal."

One family, theMacropetalichthyidæ, thus defined:

"Stegothalami with large orbits and well-arched cranio-dorsal shield. Dorso-central shield long, wide, gomphoidal, extending backward to the hinder margin of the shield and bordered by all plates save the postorbitals and marginals. Pineal funnel small and obscure."Macropetalichthys sullivantifrom Ohio Devonian rocks, andMacropetalichthys agassizifrom the Devonian of Germany, are important species of this group.

TheAsterosteidæperhaps constitute a second family in this order. The single speciesAsterosteus stenocephalusis from the Devonian of Ohio.

Arthrodira.—"Arthrognaths in which the dorsal armoring is separated into dorsal and cranial elements, the latter attached to the former movably by means of a pair of peg-and-socket joints. The interval lying between cranial and dorsal armoring does not appear to have been protected by plates, and in the median line, instead of the cranio-central of the Anarthrodires, there are separate elements, median occipital, median dorsal, and perhaps others. Suborbital plates present. Jaws of threepairs of elements. Ventral armoring of two pairs of lateral and two median elements."

Temnothoraci(τέμνω, to cut; θώραξ, thorax).—"Arthrodires whose cranial and dorsal shields are closely apposed, separated only by a transverse fissure-like interval (whence the ordinal name); interarticulation of cranial and dorsal shields little developed. Head-shield elliptical in outline as far as the line of the transverse division. The anterior rim of the shoulder-shield flattened at its sides, suggesting a rudiment of the vertical partition of the Anarthrodira. Suborbital plate is present, but takes no part, apparently, in the ventral boundary of the orbit, this being formed, as in the Anarthrodira, by the pre- and postorbital elements. Jaws, ventral armoring, and endoskeleton not definitely known."

One family,Chelonichthyidæ, thus defined:

"Temnothoraci with orbits relatively small in size and situated well forward in the head-shield. Occipital elements produced antero-posteriorly, the external occipital forming the posterior lateral angle of the head, no projection of the head occurring in the region of the marginal plate. Median occipital trapezoidal. Centrals take part in the median boundary of the orbits, and embrace the pineal plate. Median dorsal with poorly developed keel and terminal process."

Heterosteus asmussi(perhaps to be calledIchthyosauroides spinosus) is a gigantic species from the Lower Devonian of Livonia.

Allied to this species isHomostius millerifrom Scotland, celebrated as the "Asterolepis of Stromness" in Hugh Miller's "Footsteps of the Creator." Another notable species isHomostius formosissimusfrom the Lower Devonian of Russia.

Arthrothoraci.—"Arthrodires whose dorsal shield articulates with the head-roof by a conspicuous and movable peg-and-socket joint, and leaves a definite interval (unprotected?) between the two armorings. Orbits marginal, bounded inferiorly not by the suborbital element. In the head-shield the postero-lateral angles formed by the marginal plate (Phlyctænaspis?), the occipital border concave. A dorsal fin is present, supported by endoskeletal elements." Five families, the most important being theCoccosteidæ, thus defined:

"Arthrothoraci with head-shield hexagonal in outline. Median occipital trapezoidal, margins underlapped conspicuously by the external occipitals. Prefrontals meet below pineal plates, thus occluding this element from contact with centrals. The median dorsal plate elongated, terminating in an acute heavy point; no definite ventral keel; its anterior border approaches the head-shield more closely than in related families. Cranio-dorsal joint relatively small. Postero-dorso-lateral large." (?A pair of spines occurs in the pectoral region.) The best-known species isCoccosteus cuspidatus(decipiens) of the Lower Red Sandstone or Devonian of Scotland.

The family ofDinichthyidæconsists of "Arthrothoraci with stout trenchant jaws, whose cutting surfaces have worn away marginal teeth. Plates heavy. Head-shield with conspicuous lateral indentation to form dorsal border of orbit. Preorbitals separated by rostral and pineal elements, the latter passing backward between the anterior ends of the centrals. Cranio-dorsal joint conspicuous. Median dorsal shovel-shaped, nearing a stout keel with a large neck and with heavy gouge-shaped terminal. Postero-dorso-lateral relatively small in size."Dinichthys hertzeriand numerous other species are described from the Devonian and Carboniferous rocks of Ohio.

TheTitanichthyidæare "Arthrothoraci with slender edentulous jaws bearing a longitudinal sulcus. Plates squamous. Head-shield wide, with indentations to form dorsal border of orbit. Cranio-dorsal joint complete, but of relatively small size. Median dorsal with lateral border indented with rudimentary keel and with flat and rounded terminal. Antero-dorso-lateral with an area of overlap on median border."Titanichthys agassiziis a gigantic mailed fish from the Lower Carboniferous of Cleveland, Ohio.

TheMylostomidæare "Arthrothoraci with dental elements in the character of crushing plates. Cranial shield wide, rounded anteriorly, deeply indented in nuchal margin; orbital rim not apparent in dorsal aspect. Central separated from marginal."Mylostoma terrelliis based on jaws from Cleveland, Ohio.

TheSelenosteidæare "Arthrothoraci with jaws studded with cuspidate teeth; the mandibular rami rounding out anteriorly or presenting diverging tips, bearing teeth in the symphysis. Cranialshield deeply concave on lateral margins, no orbital rim here apparent. Nuchal border deeply indented. (Centrals separate from marginals.) Cranio-dorsal hinges large in size. Dorsal armoring reduced antero-posteriorly, giving an almost zone-like appearance. Dorso-median crescent-shaped, with feeble keel and knob."Selenosteus glaberis described by Dean from the Cleveland shales.

Relations of Arthrodires.—To complete our account of the Arthrodira we may here summarize Dr. Dean's reasons for separating its members from true fishes on the one hand and from the Ostracophores on the other.

"First.The Arthrodira cannot be strictly included among the Pisces. According to the definition of the latter class its members are Craniotes possessing the following characters:a, dermal defenses which in their simplest terms can be reduced to the shagreen denticles of the Elasmobranch;b, a series of definite gill-arches whose foremost elements are metamorphosed into hyoid and mandibular apparatus;c, paired fins, or their equivalents. In the first of these regards I think it can be shown that the remarkable character of the dermal plates in the Arthrognaths approaches rather that of the Ostracophores than that of the Pisces. In certain of these forms,Trachosteus, for example, the tuberculated plates are made up of inner and outer elements, each with tubercles, which denote a distinctly different mode of origin from that of any known type of fish. The absence of remains of gill-arches in the Arthrognaths would be not a serious objection to including these forms among Pisces, especially in view of the fact that cartilaginous gill-arches are rarely preserved even in favorable fossils. But that their presence is more than doubtful is indicated by the peculiar character of the 'jaws' in these forms. For the character of these structures is such as to suggest that they are not homologous with the branchial arch jaws of the true fishes, but are rather parallel structures which owe their origin to distinctly exoskeletal elements, i.e., that they were derived from dermal plates surrounding the mouth, which became mobile, and whose edges became apposed as sectorial structures. I would in this connection call attention to the fact that the 'mandibles,' 'premaxillary,' and 'maxillary'dental plates[159]were not fixed in the sense in which these elements are in the true Pisces. On the evidence of several types,Dinichthys,Titanichthys,Mylostoma,Trachosteus,Diplognathus, and other of the American forms,Macropetalichthys[160]excepted, there is the clearest proof that each element of the jaws had a considerable amount of independent movement. On account of the mobility of these elements the name Arthrognathi is suggested. Thus the mandibular rami could change the angle of inclination towards each other, as well as their plane with reference to the vertical axis. So, too, could the 'premaxillæ' be protracted like a pair of bent fingers, and it is more than probable that the 'maxillæ' had a considerable amount of independent movement. In connection with these characters it is also important to note that the blades of the 'mandible' show nowhere the faintest trace of an articular facet for attachment to the cranium. In short, the entire plan of the mandibular apparatus in these forms is strikingly unfish-like, although one will frankly confess that it is remarkable that these forms should have paralleled so strikingly the piscine conditions, to the extent of producing mandibular rami margined with teeth, and an arrangement of toothed elements on the 'upper jaw' which resembles superficially the premaxillary and maxillary structures of teleostomes, or the vomero-palatine structures of lung-fishes and chimæras.

"In the matter of paired fins there seems little evidence to conclude that either pectoral or pelvic fins were present. In spite of the researches upon these forms during the past half-century, no definite remains of pectoral fins have been described. The so-called pectoral spines described forDinichthysby Newberry, whatever they may be, certainly are not,as far as the present evidence goes, pterygial, nor are the similar structures inBrachydirus.[161]The sigmoid element, described as a 'pelvic girdle' by Smith Woodward, in Coccosteus, a structure which appears to occur in a small species ofDinichthys(?), may as reasonably be interpreted as a displaced element of the armor-plates of the trunk. InCoccosteus, as far as I am aware, it occurs in well-preserved condition in but a single specimen.

"In referring to the singular joint between the shoulder-plates and the hinder margin of the cranium Smith Woodward has called attention to one of the striking features of the group. It is one, however, which, as a functional structure, i.e., a joint, characterizes only a portion of its members; and in these the region in which vestiges of the joint are sought is overlaid and concealed by dermal plates. Such are the conditions inMacropetalichthys(with transitional characters inTrachosteusand inMylostoma). For this form a special subclass (or order) may be created which we may term Anarthrodira.

"Second.TheArthrognathicannot well be included in any other class. It would certainly be more convenient to retain the Arthrognaths among the Ostracophores, regarding them as a fourth subclass, were it not that they differ from them in so marked a way in the presence of well-marked vertebral arches, of supports for the unpaired fin, and in the possession of 'jaws.' In these regards—add to them the (probable if not certain) absence of the paired paddle-like 'spines'—they stand certainly further from the Antiarcha than these from the Osteostraci, or than the latter from the Heterostraci. It appears to me desirable, therefore, that the Arthrodira and the Anarthrodira be brought together as a separate class. Should subsequent researches demonstrate a closer affinity with the Ostracophores, the Arthrognathi can be regarded as of rank as a subclass, with the orders Anarthrodira and Arthrodira."[162]

In a recent paper Dr. Otto Jaekel unites Arthrodires and Ostracophores under the namePlacodermi. He regardsPteraspisas a larval type,Asterolepisas one more specialized. InCoccosteushe claims to find a pelvic girdle as well as a more segmented skeleton. He regards all of these as true fishes, theCoccosteidæas ancestral, related on the one hand to theCrossopterygians, and on the other to theStegocephaliand other ancestral Amphibians.

Suborder Cycliæ.—We may append to theArthrodiraas a possible suborder the group calledCycliæby Dr. Gill, based on a single imperfectly known species. Few organisms discovered in recent times have excited as much interest as this minute fish-like creature, calledPalæospondylus gunni, discovered in 1890 by Dr. R. H. Traquair in the flagstones of Caithness in Scotland. Many specimens have been obtained, none more than an inch and a half long. Its structure and systematic position have been discussed by Dr. R. H. Traquair, by Woodward, Gill, Gegenbaur, and recently by Dean, from whose valuable memoir on "The Devonian Lamprey" we make several quotations.

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