72 Bevieivs — Dr. Traquair on Silurian Fishes. 



dealt another blow at the old Agassizian conception of the importance 

 of dermal structures in classification. He has shown that * placoid 

 scales ' are as of little value for taxonoraic purposes in the Silurian 

 and Devonian periods as are 'ganoid scales' in the Tertiary and 

 existing faunas. 



Dr. Traquair's second proposition that the CoelolepidfE and Drepan- 

 aspidce possess pectoral fins, seems to me to be much more absolutely 

 an hypothesis than my contrary assertion that Cephalaspis is destitute 

 of them. So far as I understand, not a single specimen of Thelodus 

 or Lanarhia shows the slightest change in texture or arrangement 

 of its armour at the postero-lateral angles of the broad part of the 

 organism, which are described as lappet-like pectoral fins. There is 

 no more appearance of flexibility here than along any other border. 

 There are not the faintest lines of demarcation of any kind. In the 

 remarkable genus Drepanaspis the angles in question are indeed 

 strong plates, apparently without any power of motion. Nevertheless, 

 Dr. Traquair still considers them as pectoral fins, believing them to 

 be " rendered utterly functionless as fins by being enclosed in un- 

 yielding bony plates." In other words, a motile organ, originally 

 used at least for balancing, if not for progression, is assumed to have 

 lost its function, not by atrophy, but by conversion into a rigid 

 fitructure continuous with its base of support — a phenomenon for 

 which I have tried in vain to find a parallel in the animal kingdom. 

 Personally, I consider it is less hypothetical to believe that the 

 postero-lateral angles in question are not homologous with the flaps 

 in Cephalaspis, but with the hinder end of the branchial chamber 

 of the same. Both hypotheses depend on negative evidence, but 

 I prefer the latter as most reasonable. 



With respect to the flexible flap in Cephalaspis, which I regard as 

 opercular, but which Dr. I'raquair now thinks is really a pectoral 

 fin, I may repeat that we know exactly its relationships. The 

 specimens in the British Museum show that it is a flexible outgrowth 

 from the hinder margin of the roof (perhaps more accurately the 

 outer and partly upper wall) of the branchial chamber, while the 

 external outlet of this chamber is clearly a large oval opening 

 immediately beneath (within) the base of the flap. 



In conclusion, I therefore believe that Dr. Traquair's hypothesis 

 of the derivation of the Ostracodermi from an early race of 

 Elasmobranchs with a mandibular arch and paired fins, depends 

 quite as much on negative evidence as does Cope's theory that they 

 are far more primitive and best termed Agnatha. I still maintain 

 that they " are quite of a problematical character " ; but of the two 

 theories proposed for their interpretation, I think that of Cope is, for 

 the present, the less beset with diflSculties. 



A. Smith Woodwakd. 



