162 



ZOOLOGY 



SECT. 



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 (p. 76). The dermal 

 fin-rays are " horny " ; at their bases the fins are supported by 

 cartilaginous pterygiophores which are never very numerous. The 

 pelvic arch is a distinct cartilage. There is nearly always an 

 exoskeleton which is of the placoid type. The intestine has a 

 spiral or a scroll-like valve. A cloaca is present 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 con- 

 siderable breadth, and the gill-filaments are attached to them 

 throughout their entire extent. A mandibular spiracular gill is 

 only exceptionally present as a fully developed organ ; it is repre- 

 sented usually by a vestige (pseudobranch). A conus arteriosus is 



FIG. 838. Restoration of Cladoselache fyleri, lateral and ventral views. (After Dean.) 



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 ; with one possible exception 

 they are always fertilised internally. The oviducts are not con- 

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

 cavity. 



ORDER 1. CLADOSELACHII (PLEUROPTERYGII). 



Extinct Shark-like Elasmobranchs in which both pectoral and 

 pelvic fins had much wider bases of attachment than in existing 

 forms. There is an exoskeleton of small denticles. The notochord 

 was persistent : there are calcified neural and hsemal arches, but 

 no intercalary cartilages. The caudal fin is heterocercal. Claspers 

 are absent. The gill-openings were apparently protected by a 



