154 



ZOOLOGY 



SECT. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Elasmobranchii are Pisces in which the cartilaginous 

 cranium is never ossified by cartilage-bones, and in which mem- 

 brane-bones are not developed in connection either with the 

 cranium or the pectoral arch. The skull is hyostylic, except in 

 some of the Protoselachii, in which it is amphistylic. The dermal 

 fin-rays are horny ; they are supported by cartilaginous pterygio- 

 phores which are never very numerous. The pelvic arch is a dis- 

 tinct cartilage. There is nearly always an exoskeleton, which, when 

 present, is of the placoid type. The intestine has a spiral or a scroll- 

 like valve. There is a cloaca into which both the rectum and the 

 ducts of the urinary and reproductive systems open. There is 

 never an operculum in recent Elasmobranchs, and only rarely in 

 fossil forms. The inter-branchial septa are of considerable breadth, 

 and the gill-filaments are attached to them throughout their entire 

 extent. A spiracular gill is only exceptionally present as a fully 

 developed organ ; it is represented usually by a vestige (pseudo- 

 branch). A conus arteriosus is always developed ; it is rhythmically 

 contractile, and in its interior are several transverse rows of valves. 

 The optic nerves form a chiasma. The ova are very large : they are 

 usually fertilised internally. The oviducts are not continuous 

 with the ovaries, but open by wide mouths into the body-cavity. 



. 781. Restoration of Cladoselache fyleri. lateral and ventral views. (Restored, after 



Dean.) 



ORDER 1. CLADOSELACHE A. 



Extinct Shark-like Elasmobranchs in which both pectoral and 

 pelvic fins had much wider bases of attachment than in existing 



