548 SHARKS AND RAYS. 



appendages which during life were probably furnished with membranous expan- 

 sions 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 characterising the existing 

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

 rangular 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 Ischnacanthus and Diplacanthus — there are two of these fins. 



