352 



A HISTORY OF FISHES 



The third sub-class, the Ichthyotomi, is distinguished by the 

 presence of "claspers," a feature that suggests that these 

 Palaeozoic forms and the existing Selachians sprang originally 

 from a common stock. Pleuracanthus is the best-known genus, 

 and well-preserved skeletons have been found in the Carbon- 

 iferous and Permian rocks of Europe, AustraUa, and North 

 America. Although Shark-like in many respects, there are 

 certain features in which it bears some resemblance to the 

 Bony Fishes. It has a long dorsal fin, two small anals, and a 

 long tapering tail, fringed above and below by a continuous 

 caudal fin (Fig. 126). The paired fins are unique among 

 Selachians, being paddle-shaped, and with the supporting basals 

 forming a jointed central axis with the radials arranged on 

 either side (Fig. 25). Dermal denticles, as such, do not appear 



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i^^!^ .'!':'. ZlLt^j&^^ 



Fig. 126. 

 Restoration of Pleuracanthus gaudryi, X \. (After Hussakof.) 



to be present, those on the head region having been transformed 

 into roofing bones for the skull. The mouth is terminal and 

 the teeth shark-like. A curious feature is the presence of a 

 long median spine projecting from the back of the head. In 

 size, Pleuracanthus ranged from eighteen inches to six feet or 

 more, and the form of the fins and tail suggests that it was a 

 slow swimmer and lived at or close to the bottom. 



No remains that can definitely be ascribed to the members 

 of the sub-class Euselachii have yet been discovered in rocks 

 earlier than those of the Triassic period, but many of the 

 fragments of teeth and spines from Carboniferous and Permian 

 strata may have belonged to such Sharks. All the families of 

 existing Selachians, with the sole exception of the Blue Sharks 

 and their aUies [Carcharinidae] and the Sting Rays [Trygonidae), 

 include genera which have been found fossil in Cretaceous 

 rocks, and such specialised forms as the Monk-fish {Squatina) 



