-irxi X j_/i„'ivi v^xivjxviJx'V 1 21 



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 

 considerable 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 

 represented usually by a vestige (pseudobranch). 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 chiasnia. The ova are very large ; with one possible 

 exception they are always fertilised internally. The oviducts are 

 not continuous with the ovaries, but open by wide mouths into 

 the body-cavity. 



Fia. 827. — Restoration of Cladoselaclie fyleri^ lateral and ventral views. (After Dean.) 



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 

 fold of skin. The teeth are of the nature of placoid denticles. 

 The lateral line was represented by an open groove. 



This order comprise only one known representative — Cladose- 

 lache — from the lower Carboniferous rocks of America. 



