238 CLASSIFICATION OF VERTEBRATES. 



ORDER III. SELACHII. 



Elasmobranchs with the notochord more or less completely 

 replaced by vertebral centra. The neural canal completely 

 arched in by neurapophyses and intercalaria. No dermal 

 bones. Paired fins never archipterygial ; claspers always de- 

 veloped in the male. To this order belong all recent as well 

 as many fossil elasmobranchs. 



SUB-ORDER i. DIPLOSPONDYLI. 



Selachians with embolomerous vertebrae with two neural arches to each 

 myotomic centre ; a single dorsal fin ; anal fin present ; amphistylic skull, and 

 gill slits 6 or 7. Two families are recognized. The CHLAMYDOSELACHID^E 



FlG. 241. Chlamydoselachus aitguineus, after Garman. 



has an eel-like body, terminal mouth, nostrils on the dorsal surface, Cla- 

 dodtis-\\\i& teeth, and 6 gill slits. To it belongs the single genus Chlamydo- 

 selachns of the deeper portions of the northern parts of the Atlantic and 

 Pacific. In the NOTIDANID/E the body is shark-like, the spiracle is lateral, 

 the teeth differ in the two jaws. The species are viviparous. Hexanchus 

 has six gills, Heptanchus (Fig. 237) seven. 



SUB-ORDER 2. EUSELACHII. 



Vertebrae normal, branchial slits five in number, skull hyostylic. 



SECTION a ASTEROSPONDYLI. The vertebrae in this group, 

 which include the typical sharks, are asterospondylous ; two dorsals and a 

 single anal fin present. Here belong a large series of forms. In the CESTRA- 

 CIONTID^C, which is represented by a few Pacific species to-day, but which 

 was abundant in past times, the dorsal fins bear spines, while the jaws be- 

 hind bear transverse rows of pavement-like teeth. In the GALEID/E the 

 head is normal, the spiracles are small or lacking, the last gill cleft is above 



