96 STUDIES ON THE ELASMOBRANCH SKELETON, 



The first branchial arch has a short and thick copular which 

 articulates with the hinder angles of the body of the hyoid. The 

 copularia of the second arch are very long, and with those of the third 

 articulate with the anterior border of a broad common copula. 

 The latter, which is twice as broad as long, has a strongly convex 

 anterior border, and a waved concave posterior border ; its outer 

 border is directed slightly back as well as out, and with it the 

 fifth arch articulates. It presents a trace on one side of a longi- 

 tudinal division into three parts, the outer of which may represent 

 the copular of the fifth. With the posterior border of the copula 

 is articulated a short arrow-head-shaped urobranchial cartilage. 



Pectoral Fins. (Plate II., fig. 13.) 



The shoulder-girdle is characterised, in accordance with the general 

 form of the animal, by its very great breadth — its transverse being 

 much greater than its dorso -ventral extent — and also by the solidity 

 of its texture — there being no mesial connecting portion of more 

 flexible cartilage uniting the lateral halves as in other Shai-ks. 

 The mesial or transverse portion of the arch is but slightly 

 a ngulated ventrad in the middle. The articular surfaces are placed 

 on prominent processes, the long axis of which is antero-posterior, 

 or nearly so. In front of each of them, as in Cestvacion, is a very 

 prominent crest, and between the articular surface and the crest is 

 a very large foramen ; the second foramen is situated on the other 

 side of the articular process, between it and the inner edge of the 

 cartilage, at the ventral extremity of a wide shallow groove on the 

 under surface of the lateral portion of the girdle ; a little on the 

 ventral side of this opening is a conical tubercle. The lateral 

 portions of the arch are strongly incurved, but their dorsal 

 extremities are still widely separated from one another ; they end 

 in a blunt point. 



In the skeleton of the fin itself the propterygium is well- 

 developed, and consists of two nearly equal cartilages, the more 

 distal of which has a narrow accessory cartilage running along its 

 outer border. But the propterygium has no direct relation to any 



