2924 SHARKS AND RAYS 



found in the development of membrane bones overlying the original cartilaginous 

 skull; the socket of the eye being also frequently surrounded with a ring of bones 

 of similar origin. In the internal skeleton the notochord is persistent, and the 

 cartilages are superficially calcified, frequently with a granular structure. When 

 teeth are present, these are firmly fixed upon membrane bones overlying the 

 cartilages corresponding to the functional jaw 7 s of other sharks. The gill arches 

 bear a series of appendages which during life were probably furnished with mem- 

 branous expansions similar to those of the existing frill-gilled shark. In the fins 

 the cartilages of the internal skeleton are greatly reduced, and the membranous 

 portions are almost destitute of cartilaginous rays; while each of the paired and 

 most of the median fins are provided with a large spine on the front edge. The 

 tail is of the heterocercal type, and the males lack the claspers characterizing the 

 existing forms. Externally the body is covered with small and closely-arranged 

 quadrangular granules, between two series of which runs the lateral line. Three 

 families constitute the order; the first of these, as represented by the genus 

 Acanthodes, having but a single dorsal fin; while in the other two respectively 

 typified by Iscknacanthus and Diplacanthus there are two of these fins. 



