350 A HISTORY OF FISHES 



includes fusiform pelagic types as well as flattened ray-like 

 creatures. 



Even more remarkable are the members of the sub-class 

 Antiarcha, of which Pterichthys from the Old Red Sandstone of 

 Scotland and the Devonian rocks of Eifel, and Bothriolepis of 

 North America, are characteristic genera. The mouth is small, 

 the eyes are placed close together on the upper surface of the 

 head, there are no pelvic fins, and at the front end of the body 

 there is a pair of curious two-jointed and freely movable 

 appendages, not unlike the limbs of a crustacean in appearance, 

 which are entirely without parallel in vertebrate animals 

 (Fig. 125A, c). Each of these appendages is quite hollow 

 inside, and is encased in a number of bony plates. Their 

 function remains problematical, and it is by no means certain 

 that they correspond to the pectoral fins of other fishes. These 

 rather grotesque creatures, bearing a curious resemblance to 

 turtles, must be regarded as being in the nature of fantastic 

 evolutionary experiments, which flourished for a time, but were 

 doomed to extinction under stress of competition with later 

 developed types. 



The origin of the Selachians is obscure, and our knowledge 

 of the early history of this class is based only on a large number 

 of fragmentary remains and a few well-preserved skeletons. 

 The earliest traces of these fishes take the form of isolated spines, 

 or ichthyodorulites, as they are sometimes called — teeth, dermal 

 denticles and the like, from the Upper Silurian and Lower 

 Devonian rocks, interesting enough in themselves, but giving 

 no clue to the structural features of their owners. Their 

 diversity, however, is evidence that there must have existed, 

 even at this period, a wealth of genera and species, and that 

 the Selachians had already been in existence for a long time. 

 The late Professor Bashford Dean suggests that they probably 

 reached the zenith of their differentiation in the Carboniferous 

 period, "when specialised sharks existed whose varied structures 

 arc paralleled only by those of existing bony fishes — sharks 

 fitted to the most special environment; some minute and 

 delicate; others enormous, heavy, and sluggish, with stout head 

 and fin spines, and elaborate types of dentition." In the most 

 recent classification, the group of Selachians is divided into 

 five main sub-classes, but it may be necessary to erect a sixth 

 to include a remarkable Shark [Cratoselache) recently described 

 from the Carboniferous rocks of Belgium. The first three 

 sub-classes contain only extinct forms, the fourth, the Euselachii, 



