ELASMOBRANCIIS. 



237 



of living fishes ; sharks reaching, in the basking shark and blue 

 shark, a length of thirty-five or forty feet ; the rays, in Manta, 

 a breadth of fifteen feet. 



ORDER I. CLADOSELACHII (PLEUROPTERYGII). 



Notochord persistent ; neural and haemal arches slender. 

 Paired fins with basalia and radialia arranged much^as in the 

 median fins of recent fish (Fig. 188). No claspers yeTTouncT 



FlG. 240. Lateral and ventral veins of Cladoselache, restored by Dr. Dean 



Apparently a flap of skin much like an operculum covered the 

 first of the gill slits which were seven (possibly nine) in num- 

 ber. Jaws apparently hyostylic. Lateral line an open groove. 

 The only known member of the group is Cladoselache, from 

 the Waverley group (lower carboniferous) of Ohio. It is prob- 

 able that some of the fossils with similar teeth (Cladodus) 

 belong here. 



ORDER II. ICHTHYOTOMI. 



Notochord persistent ; neural and haemal arches and inter- 

 calary cartilages present. Pectoral fin archipterygial (p. 172). 

 Pelvic fins with claspers, caudal fin diphycercal. No placoid 

 scales, but the head was covered with dermal bones. The best- 

 known genus is PlenracantJins (=Xcnacant}ius, Didymodus) 

 from the carboniferous and Permian of Europe and America. 



