2J6 



ZOOLOGY 



SECT. 



like membrane -bone, the clavicle. In the remaining Ganoids and 



in Teleostei, the primary shoulder- 

 girdle is reduced in size and is 

 usually ossified by two bones, a 

 dorsal scapula and a ventral cora- 

 coid : sometimes, as in the Trout, 

 there may be an additional ossifica- 

 tion, the meso-coracoid. Additional 

 membrane bones supra-clavicle, 

 post-clavicle, &c. are added, and 

 one of them, the post-temporal, 

 serves to articulate the shoulder- 

 girdle with the skull (Fig. 815). 



In the skeleton of the pectoral fin 

 it is the Crossopterygii which ap- 

 proach most nearly to Elasmobranchs. 

 In Polypterus (Fig. 839) the basal 

 lobe of the fin is supported by a 

 rod-like ossified propterygium (Pr), 

 a broad cartilaginous mesoptery- 

 gium (MS), and an ossified meta- 

 pterygium (MT) ; to these, two 

 rows of elongated radials (Ea, Ra l ) 

 are articulated fan-wise, and these 

 in their turn give attachment to 

 the fin-rays (FS). In all the re- 

 maining orders the basalia (pro-, meso-, and metapterygium) are 

 absent, and the endoskeleton of the fin consists only of a single 

 or double row of radials (Fig. 815). 

 In Polypterus there is a vestigial 

 pelvic girdle (Fig. 839 bis, BP) in 

 the form of a small rhomboidal 

 cartilage to which the anterior 

 ends of the basalia (Bas 1 ) are at- 

 tached : thus in the structure of 

 the posterior extremities also, the 

 Crossopterygii are the most primi- 

 tive of the Teleostomi. In all the 

 remaining orders the pelvic girdle 

 is atrophied. The pelvic fin is sup- 

 ported by a single bone of variable 

 form (Fig. 816, BSTG) and re- 

 presenting a basale, i.e. a structure 

 arising from the fusion of proxi- 

 mal pterygiophores. Between its 

 posterior end and the dermal 

 rays irregular nodules, representing radials, may be interposed. 



FIG. 839. Pectoral fin of Polypterus. 

 FS. dermal rays ; MS. mesoptery- 

 gium ; MT. metapterygium ; NL, 

 nerve-foramina ; Oss. ossification in 

 mesopterygium ; Pr. propterygium ; 

 Jin. first radials ; Ra'. second radials. 

 (From Wiedersheim's Comparative 

 Anatomy.) 



Fiti. 839 b w. Pelvic fin of young- Poly- 

 pterus. Ap. part of basale ; Bas* . 

 basale ; BP. pelvic cartilages (fused in 

 adult) ; Ccp. epipubis ; Had. radials. 

 (From Wiedersheim.) 



