543 
SHARKS ANjD KAYS. 
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. 
