ELASMOBRANCHS. 



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 yet found. 



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 Pleuracanthus ( Xenacanthus, Didymodns} 

 from the carboniferous and Permian of Europe and America. 



