1906.] 



SELACHIAiSr FISHES. 



74r 



the secondary calcification forms a series of radiating ridges, 

 which do not appear to reach the periphery and are directed 

 without relation to the neural and haemal arches. The vertebral 

 column seems to have been uncalcified in the Orodontidse, 

 Oochliodontidaj, and Hybodontinre. According to Smith Wood- 

 ward, the vertebrae in Palceospinax were " cyclospondylic, 

 sometimes feebly asterospondylic," and in Synechodus " distinctly 

 asterospondylic . " 



In the Hybodontida? and Cestraciontidae, most Squalidae, and at 

 any rate some Oochliodontidse, each dorsal fin is preceded by a 



Text-fie-. 120. 



Skeleton of the dorsal fin oi Mustehis (A), Septranchias (B), Cestracion (C), 

 Squalus (D), Fristiophorus (E), and Squatina (F). (A after Thacher, 

 B and F after Mayer, E after Mivart.) 



s, spine ; b, basals ; r, radials ; in, marginals. 



spine. There can be no question as to the homology of the- 

 dorsal fin-spines in the Cestraciontidae and Squalidae, since they 



