IJNNEAN SOCIETY OF LOXDOX. 29 



PEESIDENTIAL ADDRESS, ll'iil. 



Observations on some Extinct Elasmobkaxck Fishes. 



LiNN^us was unfortunate in his treatment of the sharks, rays, 

 and chimeras, which he ultimately removed from the class of 

 fishes, and arranged among the reptiles as part ot the order 

 Amphibia Nantes. A little later the anatomists Cuvier, Johannes 

 Milller, and others, by their more extended researchess hrst 

 pointed out the true nature and relationships ot tliese hshes ; 

 while in more recent vears the morphologist, Car Gegenbaur, 

 and the embryologist, Erancis Maitland Balfour, finally led to our 

 modern conceptions of the group. The time has now arrived to 

 test their conclusions by reference to the ancestral sharks am 

 skates of which the fossil remains have been discovered and 

 studied in increasing numbers during the last three decades ; 

 and as I have had the opportunity of examnnng most ot these 

 discoveries, I propose briefly to discuss our present knowledge ot 

 the subiect. So long as most of the extinct forms were repre- 

 sented only by isolated teeth and spines, it was impossible to 

 determine satisfactorily their relationships, but now that inany 

 are known by at least parts of skeletons a detailed study ot them 

 is not in vain. Tlie teeth and spines suggested to the early 

 observers that since their first appearance in Silurian or Devonian 

 times the sharks, rays, and clumceras had always remained much 

 as they are at the present day, and could be quoted as remarkable 

 instances of persistent types. Every advance m our knowledge 

 of better-preserved fossils has tended to show that, hke all otlier 

 animals, these fishes have really evolved m many directions. 



Pleubopteeygii. 



li-aving out of consideration the earliest shark-like fislies— the 

 Acanthodian«— which developed an unique exoskeleton and appear 

 to have left no modern descendants, it may be said that the 

 Devonian sharks of the order Pleuropterygii nearly realise our 

 conception of the common ancestor. In the typical genus 

 Cladoselache^', from the Upper Devonian Waverly Shales ot 

 Ohio USA (fi"- 1 a), the slender hyomandibular seems to have 

 taken' little part°in the support of the jaws, which are of the 

 primitive amphistylic type. The mouth is terminal, and the teeth, 

 thoiuvh arranged and reproduced as in the ordinary modern sharks, 

 passVins«i^«il^le gradations of shape into the shagreen o the 

 head The notochord must have been persistent, and the cartilages 

 of the arches, so far as they have been seen, are of diagrammatic 



* B Dean, Mem. Anier. Mus. Nat. Hist. vol. ix. (1909), pt. v. ; Hiissakof & 

 Bryant, Bull. Buffalo Soc. Nat. Sci. vol. sii. (1918), p. 127. 



