140 SELACHII 



articulated to the auditory capsule by its otic process (Fig. 59, A) ; in 

 HexancJms, the articulation is looser; finally, in Cklamydoselachns, 

 the otic process, though large, does not quite reach the skull. 

 The gape of the jaws is very wide ; the hyomandibular is elongated 

 and directed backwards. The palato-quadrate bars have a basal 

 process placed far back, and are only loosely joined together in front. 

 There is a long occipito-spinal region, behind the vagus foramen, 

 the skull not being clearly marked off from the vertebral column 

 (Fig. 93). There is no triradiate rostral cartilage, and the floor of 

 the orbit is not cartilaginous. The nostrils are not confluent with 



pb. 



eb 



FIG. 100. 



Branchial arches of Heptanchus. (After Gegenbaur.) lib, basibranclii.il : rn, basiliyal ; cl l ~ 7 , 

 ceratobranchials ; cj>, fused 6th and 7th basi branchial* ; eb, epibranchial : h, hypobranchial ; 

 hy, ceratohyal ; j>6, pharyngobraiichlaL 



the mouth. The spiracle is small. There are six branchial arches 

 and slits in Hexanchus and Chlamydoselachus, seven arches and slits 

 in Heptanchus (Fig. 100). ' The slits remain widely open externally. 

 One moderately concentrated dorsal fin alone is present. There 

 is an extensive anal fin. The radials in the median fins are 

 numerous, and tend to fuse into elongated basal pieces (Fig. 48). 

 The tail is but slightly heterocercal. The notochord is largely 

 persistent, vertebral centra being feebly developed, especially in 

 Chlamydoselachus, where the notochord remains quite unconstricted 

 in the anterior region (Fig. 101). Heptanchus and the fossil 

 Notidanus, however, have well -developed centra behind, where 

 calcifications of the asterospondylous type may occur. A consider- 



