FOSSILS AND PEDIGREES 351 



includes all the existing Sharks and Rays, and the fifth, the 

 Holocephali, the Chimaeras, and their allies. 



The best-known member of the first sub-class, the Pleuro- 

 pterygii, is Cladoselache, occurring in the Upper Devonian strata 

 of Ohio. Besides being one of the more complete of the oldest 

 fossils, this Shark is by far the most primitive yet discovered, 

 and may be regarded as ancestral to most of the later types. It 

 varies in length from two to six feet, and in shape and general 

 appearance is not very unlike a modern Shark (Fig. 230). The 

 mouth, however, is terminal, and the teeth closely resemble 

 the dermal denticles of the skin. The paired fins are little 

 more than balancers, have broad bases, and may be looked 

 upon as the remnants of once continuous fin-folds {cf. p. 55). 

 The internal structure of these fins is equally primitive (Fig. 25), 

 the basal elements of the pelvics being quite separate, and the 

 pectorals are scarcely more advanced in structure. These 

 broad-based, pointed fins, taken in conjunction with the strongly 

 heterocercal tail, suggest that Cladoselache was a pelagic Shark 

 and a strong swimmer. Unlike all the existing Selachians, there 

 are no "claspers." 



Numerous and diverse fragments of the members of the 

 next sub-class, the Acanthodii, occur in the strata from the 

 Upper Silurian to the Lower Permian, but more complete 

 or well-preserved remains are comparatively rare. Each of the 

 fins of these Sharks, both paired and unpaired, has a strong 

 spine at its anterior edge, and in the genus Climatius from the 

 Lower Devonian of Forfarshire there is a row of spines on each 

 side of the body between those supporting the pectoral and 

 pelvic fins, providing additional evidence as to the existence of 

 continuous lateral fin-folds in their ancestors (Fig. 23E). The 

 body is encased in an elaborate armour, the dermal denticles 

 having been modified to form a closely fitting mosaic of minute, 

 flat, diamond-shaped scales, and some of these in the region of 

 the head have become fused to form a series of separate bony 

 plates for the protection of the skull. The teeth are few in 

 number and degenerate in structure. The known fossils are 

 all of small size, and, like the members of the preceding group, 

 show no trace of ''claspers." These Sharks cannot be regarded 

 as ancestral to any of the existing forms, but are rather a highly 

 specialised and terminal branch springing from near the base 

 of the Selachian "tree." Their extreme specialisation, coupled 

 with their small size, may have been the cause of their com- 

 paratively early extinction. 



