142 



SE LAC HI I 



able transitional region is found between the trunk and the posterior 

 diplospondylous part of the vertebral column ; here the arches are 

 double in each segment, but not the centra. 



The copulatory appendages (p. 131) resemble those of the 

 sub-orders Heterodonti and Squaliformes, but they are smooth. 



The rostrum is small in Heptanchus, scarcely at all developed 

 in Hexanchus and Chlamydoselachus. The skull and visceral arches 

 aftbrd evidence that the Notidani are the most primitive group of 

 the Selachii. 



FIG. 102. 



Dentition of Heptaii-hvs (Notiilanitx) itulieus, Guv. a, teetli in function ; >>, teetli in reserve ; 

 w, upper, and I, lower tooth of natural size. (After Giinther.) 



FAMILY CHLAMYDOSELACHIDAE. One of the most remarkable of 

 living sharks, Chlamydoselachus, is isolated in this family (Fig. 101, A). 

 The mouth is nearly terminal, and the nostrils are more lateral than 

 ventral. The edges of the gill-septa are expanded into overlapping folds. 

 The groove-like structure of the lateral line of the trunk is perhaps 

 secondary (Fig. 101, E). The teeth, with three long cusps developed 

 separately (Rose [374]), ha%-e a striking resemblance to those of the 

 Pleuracanthodii, which appears to be due to convergence. The pelvic 

 girdle is unusually long, and pierced by a double series of nerve foramina 

 (Fig. 101, G) (Garman [147]). 



Chlamydoselachus, Garman ; Pacific and Atlantic Oceans, and Pliocene 

 of Europe. 



