234 



VERTEBRATE SKELETON 



more support to the margin of the fin, this being most common in 

 skates. A fusion of adjoining radials is common. The radials of 

 sharks extend to about the middle of the fin, the distal parts of which 

 aie supported by actinotrichia (ceratotrichia), more numerous than 

 the radials. These are lacking in skates where the radials reach to 

 the border of the fin. 



Fig. 247. — A, B, pectoral and pelvic fins of Heplanchus cinereus; C, D, pectoral and 

 pelvic fins of Callorhynchus (Mivart, '79). b, basipterygium; ms, mesopterygium; mt, 

 nietapterygium; p, propterygium. 



The pectoral fins of skates (Raiae) have several peculiarities. They are 

 attached through Hfe in the primitive horizontal position, correlated with the 

 mode of swimming. More somites than usual enter the fin, which extends from 

 the pelvic fin to the head, although not all of the corresponding somites are 

 involved in it. The fin skeleton is also extended forwards by several cartilages 

 at the tip of the propterygium, each bearing radials. The most anterior of the 

 series is connected by hgament with a 'skuU-fin cartilage' (a separate part of 

 the antorbital cartilage) which, in turn, is connected with the cranium, support- 

 ing the anterior part of the fin. In some skates the middle radials of the fin 

 articulate directly with the girdle. 



Most Elasmobranchs have two basal cartilages in the pelvic fin, 

 an anterior propterygium and a posterior basipterygium (metaptery- 

 gium) the latter always articulating with the pelvic girdle and 

 usually with the propterygium also when large. In Chlamydoselachns 

 (fig. 245), Xenacantlms, etc., some of the anterior radials are attached 

 directly to the pelvis. Many genera have the fasipterygium con- 

 tinued distally by a .series of radial-Hke joints. The pelvic fins of 



