176 EVOLUTION OF ELASMOBRANCHS vi. 2-3 



Superclass Gnathostomata (cont.) 



Subclass 2. Bradyodonti. Devonian-Recent 

 *Order 1. Eubradyodonti. Devonian-Permian 



*HeIodus 

 Order 2. Holocephali. Jurassic-Recent 



Chimaera 



The elasmobranchs form a very compact group of fishes, nearly 

 always marine and of predaceous habit, having a great quantity of 

 urea in the blood, with no bone in the skeleton, no operculum over 

 the gills, and no air-bladder. The tail is usually heterocercal. The 

 pectoral fin is anterior to the pelvic and the latter is usually provided 

 with claspers, fertilization being internal. The body is more or less 

 completely covered with placoid scales (denticles) and these are 

 specialized in the mouth to form rows of teeth. The intestine is short 

 and provided with a spiral valve. The typical cartilage-fishes with 

 these characters may be placed in the subclass Selachii, to distinguish 

 them from an early aberrant offshoot the Bradyodonti, represented 

 today by the peculiar creature Chimaera (p. 184). 



3. Palaeozoic elasmobranchs 



The selachians are among the most numerous of the various pre- 

 datory animals in the sea. There have, however, been many side- 

 branches of the main shark line and we may now survey the history 

 of the group from its first appearance. The characters we have used 

 in our definition mark the elasmobranchs off from the earliest-known 

 gnathostomes, the acanthodians and other placoderm types (Fig. 1 1 1), 

 which we shall consider later (p. 186). Presumably the elasmobranchs 

 were derived from some placoderm, but the earliest evidence of the 

 existence of true sharks is in the form of isolated teeth and scales from 

 Middle Devonian deposits, and the earliest type about which full 

 information exists is *Diade?nodus from the Upper Devonian, 'an 

 early and not distant offshoot from the primitive Chondrichthyan 

 stock, the main line of which led through *Ctenacanthus and the 

 hybodonts to the modern elasmobranchs ; *Cladoselache is a specialized 

 side-line of this main stock and is not an appropriate ancestral type 

 for the Chondrichthyes' (Harris). The teeth of *Diademodus are 

 many-cusped and resemble the scales more closely in sculpturing 

 than in other primitive sharks. The jaw suspension was amphistylic 

 and the notochord unconstricted. The pectoral fin was continuous 

 posteriorly with the body wall and there was no well-developed 



