FINS OF ELASMOBEANCHS. 471 



appears to me to show how slight a modification might have produced that separation 

 and aggregation of the dorsal radials which now exists. It is evident how little change 



Fig. 3. 



Figure of the dorsal-fin cartilages of Adpenser, after Thacher. 



would be needed to make the three preaxial radials continuous with three neural spines 

 subjacent to them. Again, let the neural spines of Adpenser be somewhat more 

 developed, and let the preaxial parts of the radials coalesce together (as in Notidanus) 

 and then v^ith the neural spines, and we at once get the condition presented by Pris- 

 tiophorus and Pristis. 



But what are we to consider the nature of the skeleton of the caudal fin to be ■? 

 We have again and again found a discordance between its dorsal and ventral portions. 

 In Elasmobranchs the dorsal cartilaginous radials are ever more numerous than the 

 vertebrae, are unconformable to them, and small, while in the ventral portion of the fin 

 they conform in number to the vertebrae (however at first apparently discordant from 

 diflierences of size) and are large. In the former they are not continuous with the 

 vertebrae ; in the latter (ventral portion) they are so. This seems to point to a differ- 

 ence in nature between the dorsal and ventral portions of the caudal fin in, at least 

 most, Elasmobranchs. 



This distinctness of nature seems pointed to also in Polyodon, where the dorsal 

 part of the caudal fin is supported by bones homotypal with the dorsal-fin supports, 

 while its ventral part is quite devoid of such structures (thus differing from the 

 adjacent anal fin), and has for supports extensions of the axial skeleton only, to which 

 fin-rays are distally attached. 



As to the condition of this part in Teleosteans generally, I will not as yet venture 

 to say any thing, except that it is plain that in such forms as Murcena the dorsal and 

 ventral parts of the caudal are similar in nature and homotypal with ordinary dorsal 

 and anal fins. 



If the neural spines of vertebrae are ingrowths from the dorsal azygos fin-fold, 

 one would naturally expect that the haemal spines would be similar ingrowths from 



VOL. X. — PAET X. No. 5. — February 1st, 1879. 3 s 



