CLASSIFICATION OF YERTEBRATA 



C hlamydoselach e anguineus, approaching the Proselachii. 



Japan, Azores, Norway. 

 Heptanchu s. Hexanchus, Pristis, Scylliuni, Mustelus .^*^, 

 Galeus, Squatina, Zygaena, Cestracion, etc. 

 2. Sub-order KAIAE. Pectoral fins fused with and 

 surrounding the sides of the head. 



Beginning clearly in the Jurassic strata. 

 Eaia , Trygon, Myliobates, Torpedo , etc. 



II. Division — ACANTHODI, Agassiz. With five Elasmo- 

 branch gill-clefts, but each with an external fringe-flap. Mouth 

 sub-terminal. 



Vertebrae acentrous. — Without mixipterygia. 

 Paired and median fins with a strong dermal spine. 

 Large dermal bones on the jaws ; dermal armature of trunk 

 and cranium consisting of small granules. 



Ac anthode s. From Devonian to Permian, Europe. 

 Upper Devonian : pelvic fins almost as large as the 

 pectorals, and placed midway between pectorals and anal fin. 

 Lower Carboniferous : pelvics reduced in size. 

 Upper Carboniferous : pelvics much reduced, and placed 

 far forwards towards the enlarged pectorals. 



Lower Permian : pelvics insignificant ; pectorals enormous, 

 and closely approximated towards each other. 



Chiracanthus. Old Bed, Scotland ; both fins well de- 

 veloped. 

 Diplacanthus. Old Eed, Scotland ; with clavicle and 

 cleithrum. 



III. Division — HOLOCEPHALI, Mueller. Palato-quad- 

 rate bar fused with the cranium. 



Vertebrae acentrous ; numerous calcified rings in the 

 notochordal sheath. 



Agreeing with Elasmobranchi : males with mixipterygia ; 

 absence of membrane bones ; conus arteriosus with three series 

 of valves. 



Agreeing with Tectobranchi : four gill-clefts, with one large 

 operculum ; the gill-septa are thin and incomplete, no longer 

 reaching the surface ; with a few large teeth resembling those 

 of Dipnoi ; mouth subterminal. 

 Ptyctodus. Devonian. 



