2i6 COMPARATIVE ANATOMY 



sinuses. The fourth and fifth pharyngo-branchials are joined 

 together. The form and position of the branchial arches will 

 be best understood by reference to fig. 52. The posterior 

 edges of all the ceratobranchials except the last are furnished 

 with cartilaginous gill-rays. 



Under the head of the appendicular skeleton are reckoned 

 the cartilaginous supports of the median and paired fins. 

 The paired fins are attached to cartilaginous supports or 

 girdles which connect them with the trunk. The pectoral 

 girdle consists of two half-hoops of cartilages, of which the 

 lower ends are fused together in the mid-ventral line, while 

 the upper ends remain separate and are imbedded in the 

 muscles of the trunk. The ventral moiety of each half-hoop 

 is called the coracoid, the dorsal moiety the scapula, and at 

 the point where the coracoid passes into the scapula the hoop 

 is thickened and bears a triple facet for the articulation of the 

 basal cartilages of the fin. The ventral conjoined ends of the 

 coracoids are flattened and produced forward into a prominent 

 process which strengthens the floor of the pericardial cavity. 

 The skeleton of the pectoral fin consists of three basal cartilages 

 known as the propterygium, mesopterygium, and metaptery- 

 gium, the last named being the largest, and forming a con- 

 siderable part of the inner border of the fin. As may be seen 

 in fig. 52, the propterygium is succeeded distally by a single 

 fin-ray, consisting of a plate of cartilage, the mesopterygium is 

 similarly succeeded by a single broad ray divided into two or 

 three distal lobes, but the metapterygium bears a number of 

 narrower cartilaginous rays. The proximal fin-rays are suc- 

 ceeded by two or more rows of polygonal plates of cartilage, the 

 outer row supporting the horny dermal fin-rays or actinotrichia 

 which support the greater part of the expanse of the fin. In the 

 posterior or pelvic limbs the girdle is reduced to a horizontal 

 bar of cartilage lying just in front of the cloaca. The basal 

 skeleton of the fin is a curved rod, the basipterygium, the 

 front end of which articulates with the pelvic girdle. The 

 fin-rays are slender cartilaginous segmented rods articulated 

 to the outer border of the basipterygium, with the excep- 

 tion of the first, which articulates directly with the pelvic 

 girdle. The claspers of the male are provided with stout 

 skeletal cartilages, articulated to the hinder ends of the basi- 

 pterygia. 



