146 



SELACHII 



radials, which are much concentrated, and fused proximally to a large 

 basal. In Hybodus the spines have a serrated posterior edge apparently 

 formed by the fusion on to the spine of a double 

 series of denticles (Fig. 106). 



The nostrils are confluent with the mouth. A 

 strong supraorbital crest is developed, bearing two 

 pairs of specially enlarged spines in the male 

 Hybodus, Acrodus, and Asteracanthus. The spiracle 

 is very small, and placed below the eye. The base 

 of the pectoral fin grows forward below the last three 

 branchial slits. The pectoral girdle is very powerful. 

 The teeth become differentiated into grinding plates 

 at the sides, but do not fuse. The egg - case of 

 Heterodontus is provided with a peculiar spiral lamina 

 (Fig. 91, H). 



Hybodus, Ag. ; Triassic and Jurassic, Europe. 



Sphenacanthus, Ag., Tristychius, Ag., from the 



FIG. 105. 



Portion of the lower jaw and teeth of Aster- 

 acanthus (Strophodus) medius, Owen ; Great 

 Oolite, Caen. (From Brit. Mus. Guide.) 



Fio. 106. 



Spine of Hybodus 

 subcarinatus, Ag. 

 (After Owen.) 



Carboniferous of Europe. Orodus, Ag. ; Carboniferous of Europe and 

 America. Wodnika, von Munst. ; Permian. Acrodus, Triassic to Cretaceous. 

 Asteracanthus (Strophodus), Ag., Palaeospinax, Eg., Cestracion, Cuv. ; Jurassic, 

 Europe. Synechodus, A. S. W. ; Cretaceous, Europe and New Zealand. 

 Heterodontus (Cestracion}, Blainv. ; Port Jackson shark, Pacific (Fig. 103). 



Family COCHLIODONTIDAE. Incompletely known fossils chiefly from 

 Carboniferous strata (Davis, Owen, A. S. Woodward). Helodus had an 

 anal fin and a spinous dorsal. The teeth resemble those of Cestracion, 

 but are farther modified. Not only do neighbouring teeth tend to fuse 

 at the sides, but also successive generations ; so that paired crushing plates 

 are produced, the worn outer edge of which is coiled inwards. 



Helodus, Ag., with detached teeth ; Psephodus, Ag., with the two outer 

 rows fused; Pleuroplax, A. S. W. ; Deltodus, Ag. ; Cochliodus, Ag., with a 



