58 



The Skeleton of the Fish 



tlie back-bone at a distance behind the head. To this cartilage 

 three smaller ones are attached, forming the base of the pectoral 

 fin. These are called me so pterygium, proterygium, and iiieta- 



pteryginm, the first named 

 being in the middle and 

 more distinctly basal. 

 These three segments are 

 subject to much varia- 

 tion. Sometimes one of 

 them is wanting; some- 

 times two are grown to- 

 gether. Behind these the 

 fin-rays are attached. In 

 most of the skates the 

 shoulder-girdle is more 

 closely connected with 

 the anterior vertebra, 

 which are more or less 

 fused together. 



The pelvis, remote 

 from the head, is formed, 

 in the shark, of a" single 

 or paired cartilage with 

 smaller elements at the 

 base of the fin-rays. In 

 the males a cartilaginous 

 generative organ, known 

 as the clasper, is attached 

 to the pelvis and the 

 ventral fins. In the 

 Elasmobranchs the tail 

 vertebra; are progressively smaller backward. If a caudal fin 

 is present, the last vertebra; are directed upward (heterocercal) 

 and the greater part of the fin is below the axis. In other forms 

 (sting-raysj the tail degenerates into a whip-like organ {lepto- 

 ccrcal), often without fins. In certain primitive sharks (Ichth^^o- 

 tomi), as well as in the Dipnoi and Crossopterygii, the tail is 

 diphycercal, the vertebra; growing progressively smaller back- 

 ward and not bent upward toward the tip. 



Fig. 47. — Shoulder-girdle of a Flounder, Para 

 lirhtlojs californicus (Ayres). 



