PRIMEVAL SHARKS. 



23 



elongated (see fig. 10, and the enlarged drawing 9 at the top of 

 Wall-case 1). Two dorsal fins are present, but no anal fin. 

 The caudal portion of the vertebral axis is strongly upturned, and 

 distinctly hinged upon the rest of the vertebral column, but the 

 outline of the tail is symmetrical about a horizontal plane. A 



Fig. 10. — An extinct and archaic Shark, Cladoselache fyleri, 



side and ventral views, restored. 

 (From Bridge, Camb. Nat. Hist., vii, 1904, after B. Dean.) 



short dermal expansion forms a horizontal keel on each side of the 

 base of the tail. No calcifications are recognisable in the sheath 

 of the notochord ; the skeletal cartilages are calcified in the form 

 of cubes, closely fitting like a mosaic. The teeth are of various 

 forms, each with a principal cusp and a variable number of lateral 

 cusps. The eye has a ring of small dermal plates; the remainder 

 of the body is covered with minute denticles. 



ACANTHODIDES (Acanthodian Sharks). 



The Acanthodian fishes are Palteozoic Sharks of small size, Climatius. 

 rarely exceeding one foot in length. The restoration of Climatius 

 shown (11) is about four times the natural size (linear). The 



