APPENDICULAR SKELETON FISHES 



233 



Several 



Fig. 246. — Inside of 

 half of pectoral girdle 

 of Myliobatis (Gegen- 



in the body wall and sometimes (Acanthias, Xenacanthus, etc.) bears 

 a suprascapula. Its dorsal border is free in sharks; in skates is con- 

 nected (by ligament or directly) with the anterior vertebra? 

 genera (Acanthias, Heptanchus) have a distinct 

 cartilage (possibly presternal, p. 52 between the 

 coracoid regions of the two sides. 



A few sharks have the halves of the pelvic 

 girdle separate through hfe; in only a few does it 

 remain a broad plate; elsewhere it is a bar with the 

 articular surface on the posterior side to accom- 

 modate the posteriorly directed fins. Skates (fig. 

 248, A) often have a prepubic process a httle 

 medial to the acetabular surface and extending 

 downwards and inwards. There is usually a single 

 obturator foramen, but in Chlamydoselachus (fig. 

 24s) and Holocephala there are several variously baur, '65. modified) 



^^ ^ c, s, coracoid and 



arranged. scapular regions; g. 



In sharks whose development is known, after ^"^^i^. ,^Jrve." '^ 

 the union of the pelvic cartilages of the two sides, 

 the pelvic girdle is at first U-shaped with well developed ihac region, 

 which tends to disappear while the ischio-pubic part becomes a 

 straight bar. The ihac portion remains large in skates (fig. 248, ^) 

 and Holocephals. 



Since their functions are much alike, pectoral and pelvic fins 

 of sharks are very similar. Both have jointed radials which either 

 articulate with the girdle or with one, two or three basal cartilages, 

 these articulating in turn with the girdles. The pectoral fin is the 

 larger and there are some other differences. Some sharks {e.g., 

 Scymnus) have a single basal cartilage (basipterygium) on the post- 

 axial side to which all of the radials are attached. Other sharks, 

 with larger pectorals, have more numerous radials, some of which 

 may articulate directly with the girdle. In such cases the basal 

 part of at least one anterior radial is enlarged, forming a second 

 basal element of the fin, the propterygium (fig. 247); in still others 

 the propterygium is clearly the fused basals of several rays; in these 

 the other basal is called the metapterygium. A farther specialization 

 results in a mesopterygium between the pro- and metapterygia 

 (fig. 247,.4). The radials are usually divided transversely, the number 

 of articles varying, and sometimes the distal ends are split, giving 



