804 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



the former the radials are more numerous than the vertebrae and 

 unconformable to them, while in the latter they are equal in 

 number to the vertebras and continuous with them. "This," he 

 goes on to say, "seems to point to a difference in nature be- 

 tween the dorsal and ventral portions of the caudal fin, in at 

 least most Elasmobranchs." He further points out that Polyodon 

 resembles Elasmobranchs. As to Teleostei, he does not express 

 himself decidedly except in the case of Murcena, to which we 

 shall return. 



Mivart expresses himself as very doubtful as to the nature of 

 the supports of the caudal fin, and thinks " that the caudal fin of 

 different kinds of Fishes may have arisen in different ways in 

 different cases." 



An examination of the ventral part of the caudal fin in various 

 Ganoids, Teleostei, and Elasmobranchii appears to us to shew 

 that there can be but little doubt that, in the majority of the 

 members of these groups at any rate, and we believe in all, the 

 same distinction between the ventral lobe of the caudal fin and 

 the remaining unpaired fins is found as in Lepidosteus. 



In the case of most Elasmobranchii, a simple inspection of 

 the caudal fin suffices to prove this, and the anatomical features 

 involved in this fact have usually been recognized ; though, in the 

 absence of embryological evidence, the legitimate conclusion has 

 not always been drawn from them. 



The difference between the ventral lobe of the caudal fin and 

 the other fins in the mode in which the fin-rays are supported is 

 as obvious in Chondrostean Ganoids as it is in Elasmobranchii ; 

 it would appear also to hold good for Amia. Polypterus we have 

 had no opportunity of examining, but if, as there is no reason to 

 doubt, the figure of its skeleton given by Agassiz (Poissons 

 Fossiles) is correct, there can be no question that the ventral lobe 

 of the caudal fin is supported by the haemal arches, and not 

 by interspinous bones. In Calamoicthys, the tail of which we 

 have had an opportunity of dissecting through the kindness 

 of Professor Parker, the fin- rays of the ventral lobe of the 

 true caudal fin are undoubtedly supported by true haemal 

 arches. 



There is no unanimity of opinion as to the nature of the 

 elements supporting the fin-rays of the caudal fin of Teleostei. 



