372 



Bashford Dean hiemorial Voluyne 



Chlamydoselachus shows a large secondary mesopterygium, as in Braus's figure, and the 

 primary mesopterygium also resembles that figured by Braus. The posterior radials are 

 segmented to form no more than three rows of segments. At the extreme posterior ends 

 of the fins shown in the various figures there are individual differences. 



In connection with his study of the development of paired fins, Sewertzoff (1926, 

 p. 547) states: 



It is now generally accepted that the skeleton of the fins of the lowest cartilaginous 

 fishes (Chondropterygii) has developed from metamerically disposed rays, and that the basal 

 cartilage of the free parts of the fin, i.e., the pro-, meso-, and the metapterygium, as well as 

 the girdles, were formed by the concrescence or fusion of the proximal segments of these rays. 

 But this view may not be considered settled, and, looking over the Kterature of this question, 

 we see that many writers, who accept the theory of the [metameric] origin of the paired 

 fins, pass over in silence the question of the primitive structure of their skeletons or express 

 themselves on that subject with considerable caution. 



In the pectoral fin skeletons of both Cladodus neilsoni Traquair (Text 'figure 39) 

 and Symmorium reniforme Cope (Text-figure 40) there is only one basal that can be 



Text 'figure 39. Text-figure 40. 



Pectoral girdles and fin skeletons of two fossil sharks, Cladodus and Symmorium. 



Text-figure 39. Endoskeleton of the pectoral fin of Cladodus niehoni Traquair. 



B, basal piece; BL, fracture line; Mt, metapterygium; R, radial; S, furrow in outer proximal margin of 



the metapterygium. 

 From Braus, 1902, Fig. 2; after Traquair, 1897, Fig. 1, pi. IV. 



Text-figure 40. Fragment of a pectoral fin skeleton of Symmorium reniforrne Cope. 



B, basal piece; Mt., metapter>'gium; R, some small radials at the distal end. 



From Braus, 1902, Fig. 3; after Cope, 1895. Fig. 1, pi. VIII. 



