September 20, 1900] 



NATURE 



505 



I 



to their immediate origin, no intermediate forms having been 

 found connecting them more closely with either the Hetero- 

 straci or the Osteostraci. In the possession of bone lacunce and 

 of a dorsal fin they have a greater resemblance to the latter, but 

 it may be looked upon as certain that they could have had no 

 direct origin from that group. 



As regards the Ostracodermi as a sub-class, they become 

 extinct at the end of the Devonian epoch, and cannot be credited 

 with any share in the evolution of the fishes of more recent 

 periods, not even if we restore the Coccosteans or Arthrodira to 

 their fellowship. To the latter most enigmatical group, which I 

 shall still continue to look upon as fishes, I shall make some 

 reference further on. 



Coming now to say a word regarding the Elasmobranchii, it 

 is plain from the fin-spines found in Upper Silurian rocks that 

 they are of very ancient origin, and that if we only knew them 

 properly they would have a wonderful tale of evolution to tell. 

 But their internal skeleton is from its nature not calculated for 

 preservation, and for the most part we only know those creatures 

 from scattered teeth, fin-spines and shagreen, specimens show- 

 ing either external configuration or internal structure being rare, 

 especially in Palaeozoic strata. But from what we do know, 

 there is no doubt that the ancient sharks were less specialised 

 than those of the present day, and that the recent Notidanids 

 still preserve peculiarities which were common in the Selachii of 

 past ages. 



If we ask whether the fossil sharks throw any light on the dis- 

 puted origin of the paired limbs, whether from the specialisation 

 of right and left lateral folds, or whether that type of limb 

 called " archipterygium " by Gegenbaur, consisting of a central 

 jointed axis with pre- and post-axial radial cartilage at- 

 tached, was the original form, I fear we get no very definite 

 answer from Elasmobranch palaeontology. The paired fins of 

 the Upper Devonian shark, Cladoselache, as described by Bash- 

 ford Dean, Smith Woodward and others, seem to favour the 

 lateral fold theory, and Cope pointed to the right and left series 

 of small intermediate spines which in some Lower Devonian 

 Acanthodei (Parextis and Climatius) extend between the 

 pectorals and ventrals as evidence of a former continuous lateral 

 fin. So also, if I am right in looking on the lateral flaps of the 

 CrelolepidjE as fins, the evidence of these ancient Ostracodermi 

 would be in the same direction. 



But, on the other hand, we have the remarkable group of 

 Pleuracanthidre, extending from the Lower Permian back to 

 the Upper Devonian, in which the paired fins are represented 

 by an "archipterygium" which in the pectoral at least is 

 biserial, 



From this biserial "archipterygium " in the Pieuracanthidae, 

 Prof. A. Fritsch, ten years ago, ^derived the tribasal arrangement 

 of modern sharks, much according to the Gegenbaurian method, 

 effecting, however, a compromise with the lateral fold theory 

 by assuming that the Pleuracanth form originated from one, 

 consisting of simple parallel rods, like that described in 

 Cladoselache. 



In my description of the pectoral fin of the Carboniferous 

 Cladodus Neihoni'- 1 have shown that the cartilaginous structures 

 apparently present an uniserial archipterygium intermediate be- 

 tween the arrangement in Plenracanthtis and that in the modern 

 sharks, but I felt compelled to acknowledge that the specimen 

 might also be interpreted in exactly the opposite way, namely, 

 as an example of a transition from the " ptychopterygium" of 

 Cladoselache to the Pleuracanth and Dipnoan limb. And so in 

 fact this fin of Cladodus is claimed in support of their views by 

 both parties in the dispute. 



When we add that Semon emphatically denies that there is 

 any proof for considering that the pectoral fin of Cladoselache is 

 primitive in its type,' and that Campbell Brown, in his recent 

 paper on the Mesozoic genus Hybodus,* supports Gegenbaur's 

 theory, it will be seen that Elasmobranch palaeontology has not 

 as yet uttered any very clear or decided voice on the question as 

 to whether the so-called archipterygium is the primary form of 

 paired fin in the fish, or only a secondary modification. We 

 shall now inquire if we can obtain any more light on the subject 

 from the Crossoplerygii and Dipnoi. 



1 " F.iuna der Gaskohle und der Kalksteine der Permformation B&hmens," 

 vol. iii. Pt. i. (Prague, 1890), pp. 44-45. 



2 Trans. Geol. Soc. (Glasgow), vol. xi. Pt. \. 1897, pp. 41-50. 



3 " Die Entwickelung der paarigen Flossen des Ceratodus Forsteri." 

 (Jena, 1898.) 



•♦ " Ueber das Genus Hybodus und seine systematische Stellung,' 

 PalaeoHtographica, vol. xlvi. 1900. 



NO. 1612, VOL. 62] 



The Crossopterygii are a group of Teleostomous fishes, charac- 

 terised externally by their jugular plates and lobate paired fins, 

 and represented in the present day only by the African genera 

 Polypterus and Calamoichthys, which together form the peculiar 

 family Polypteridre. The Crossopterygii appear suddenly in the 

 middle of the Devonian period, their previous ancestry being 

 unknown to us. 



Four families ' are known to us in Palaeozoic times— the Osteo- 

 lepidae, Rhizodontidae, Holoptychiidse and Coelacanthidae, but 

 it is only with the first three that we have at present to deal. 

 The Osteolepidai and Rhizodontidae, which appear together in 

 Middle, and die out together in Upper Palaeozoic times, re- 

 semble each other very closely. In both we have the paired 

 fins, more especially the pectoral, obtusely or subacutely lobate ; 

 there are two separate dorsal fins, one anal, and the caudal, 

 which is usually heterocercal, though in some genera it is more 

 or less diphycercal. In both the teeth are conical and have the 

 same complex structure, the dentine being towards the base 

 thrown into vertical labyrinthic folds, exactly as in the Stego- 

 cephalian Labyrinthodonts, and this along with the lung-like 

 development of the double air-bladder in the recent Polypteridae 

 has given rise to the view that from these forms the Stego- 

 cephalia have originated. The nasal openings must have been 

 on the under surface of the snout, as in the Dipnoi. 



Of these two so closely allied families we must conclude that 

 the Osteolepidae are the more primitive, as in them the scales 

 are acutely rhombic and usually covered with a thick layer of 

 ganoine, while in the Rhizodontidae they are rounded, deeply 

 imbricating, and normally devoid of the ganoine layer, which, 

 however, occasionally recurs on the scales of Rhizodopsis and 

 the fin-rays of Gyroptychius. 



What then of the structure of the paired fins ? Fortunately 

 in the Rhizodont genera Tristichoplerus and Eusthenopteron the 

 internal skeleton of the lobe was ossified, and what we see 

 clearly exhibited in the pectoral of some specimens is striking 

 enough. We have a basal piece attached to the shoulder-girdle 

 and followed by a median axis of four ossicles placed end to 

 end. The first of these shows on its postaxial margin a strong 

 projecting process, while to its preaxial side, close to its distal 

 extremity, a small radial piece is obliquely articulated, and a 

 similar one is joined also to the second and third segments of 

 the axis. The arrangement in the ventral fin is essentially 

 similar. 



In fact, we have in the Rhizodontidae a short uniserial 

 " archipterygium," and the question is. Has this been formed 

 by the shortening up and degeneration o.f an originally elongated 

 and biserial one, or on the other hand do we find here a condi- 

 tion in which the stage last referred to has not yet been 

 attained ? This question is inseparable from the next, whether 

 the Rhizodonts or the Holoptychians form the most advanced 

 type? 



The Holoptychiidae resemble the Rhizodontidae extremely 

 closely in their external head-bones, in their rounded, deeply 

 imbricating scales, and in the form and arrangement of their 

 median fins. But the teeth show a more complex and special- 

 ised structure than those of the Rhizodontidae ; the simple 

 vertical vascular tubes formed by the repeated folding of the 

 dentine in that family being connected by lateral branches 

 around which the dentine tubules are grouped in such a way as 

 to give rise in transverse sections to a radiating arborescent 

 appearance; hence the term " dendrodont." In this respect, 

 then, the Holoptychiidae show an advance on the Rhizodontidae 

 —what then of the paired fins ? While the ventral remains sub- 

 acutely lobate, as in the previous family, the pectoral has now 

 assumed an elongated acutely lobate shape, with the fin-rays 

 arranged along the two sides of a central scaly axis exactly as 

 in the Dipnoi ; and though the internal skeleton has not yet 

 been seen, yet, judging by analogy, we cannot escape the belief 

 that it was in the form of a complete biserial "archipterygium." 



What, then, is the condition of affairs in the oldest known 

 Dipnoan ? 



The oldest member of this group with whose configuration we 

 are acquainted is Dipterus, which likewise appears in the 

 middle of the Devonian period simultaneously with the Osteo- 

 lepidre, Rhizodontidae and Holoptychiidae. In external form it 

 closely resembles a Holoptychian, having a heterocercal caudal 

 fin, two similarly placed dorsals, one anal, and circular imbri- 

 cating scales, which, however, have the exposed pirt covered 

 1 Five, if we include the singular and still imperfectly known Tarrasiidie 

 of the Lower Carboniferous. 



