774 EEPORT — 1900. 



specialised forms, must fall to the ground. And from the close resemblance of these 

 scales of Thelodus to Elasmobranch shagreen bodies — for forty-five years they had 

 been, by most authors, actually referred to the Selachii — I concluded that the 

 Ccelolepidse owed their origin to some form of primitive Elasmobranchs. That 

 is, however, not in accordance with the view of the late Professor Cope, that the 

 Ostracodermi are more related to the Marsipobranchii, and that, from the apparent 

 absence of lower jaw, they should be placed along with the last-named group in a class 

 of Agnatha, altogether apart from the fishes proper. And Dr. Smith Woodward, 

 who is inclined to favour Cope's theory, has expressed his view that the similarity 

 of the Coelolepid scales to Elasmobranch shagreen is no proof of an Elasmobranch 

 derivation, but that such structures, representing the simplest form of dermal hard 

 parts, may have originated independently in far distant groups.^ Knowing what 

 we do of the occurrence of strange parallelisms in evolution, it would not bo safe 

 to deny such a possibility. But as to a Marsipobranch affinity, I would point out 

 that the apparent want of lower jaw among the hard parts which nature has pre- 

 served for us is no proof of the absence of a Meckelian cartilage among the soft parts 

 which are lost to us for ever ; and also, as Professor Lankester has remarked, that 

 there is no evidence whatever that any of the creatures classed together as Ostraco- 

 dermi were monorhinal like the Lampreys. The only fossil vertebrate having a 

 •single median opening, presumably nasal, in the front of the head is Pcdaio- 

 spondylus, but, whatever be the true affinities of this little creature, at present the 

 subject of so much dispute, I think we may be very sure that it is not an 

 Ostracoderm. 



The Devonian ' Antiarcha ' or Asterolepida, of which Pterichthi/s is the best 

 known genus, are also usually placed in the Ostracodermi, with which they agree 

 in the possession of a carapace of bony plates, in the absence of distinct lower jaw 

 or teeth, in the non-preservation of internal skeleton, and in having a scaly tail 

 furnished with a heterocercal caudal fin, and, as in the Cephalaspidae, also with a 

 small dorsal. But they have in addition a pair of singular jointed thoracic limbs, 

 evidently organs of progression, which are totally unlike anything in the Osteostraci 

 or in the Heterostraci, or indeed in any other group of fishes. These limbs are 

 covered with bony plates and hollow inside; but though I once fancifully compared 

 them in that respect with the limbs of insects, I must protest strongly against this 

 expression of mine being quoted in favour of the arthropod theory of the deriva- 

 tion of the Vertebrata ! 



'N'or do I think that there is any probability in the view published by Simroth 

 nine years ago," namely, that Pterichthys may have been a land animal which used 

 its limbs for progression on dry ground, and that the origin of the heterocercal tail 

 was the bending up of the extremity of the vertebral axis caused by its being 

 dragged behind the creature in the act of walking. That view was promulgated 

 before the discovery of the membranous expanse of the caudal fin in this 

 genus. 



But though the Asterolepida are apparently related to and inclusible in the 

 Ostracodermi, the geological record is silent as to their immediate origin, no inter- 

 mediate forms having been found connecting them more closely with either the 

 Heterostraci or the Osteostraci. In the possession of bone lacunjs 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 upou 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 



' Geol. Mag., March 1900. 



^ ' Die Entsteliang der Landthiere,' Leipzig, 1891. 



