Notices of Memoirs — Dr. JR. H. Traquair^s Address. 469 



simplest form of dermal hard parts, may have originated inde- 

 pendently in far distant groups.' Knowing what we do of the 

 occurrence of strange parallelisms in evolution, it would not be 

 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 preserved 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 Ostracodermi were monorhinal like the Lampreys. The 

 only fossil vertebrate having a single median opening, presumably 

 nasal, in the front of the head is Palaiospondybis, 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 Cephalaspidse, 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 derivation of the Vertebrata ! 



Nor do I think that there is any probability in the view published 

 by Simroth nine years ago,- namely, that Pterichihys 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 pro- 

 mulgated before the discovery of the membranous expanse of the 

 caudal fin in this genus. 



But though the Asterolepida are apparently related to and in- 

 clusible in the Ostracodermi, the geological record is silent as to 

 their immediate origin, no intermediate forms having been found 

 connecting them more closely with either the Heterostraci or the 

 Osteostraci. In the possession of bone lacunaa 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 



1 Geol. Mag., March, 1900. 



^ " Die Entstehung der Landthiere" ; Leipzig, 1891. 



