xm 



PHYLUM CHORDATA 



203 



COR.), situated above the coracoid and anterior to the scapula. 

 Externally to these is found a very large investing bone, the clavicle 

 (CL.), extending downwards under the throat : its dorsal end is 

 connected by means of a supra-clavicle (S. CL.) to a forked bone, 

 the post-temporal (P.TM.), one branch of which articulates with 

 the epiotic, the other with the pterotic process. To the inner 

 surface of the clavicle are 'attached two flat scales of bone 

 (P. CL'.), with a slender rod-like post-clavicle (P. CL.) passing 

 backwards and downwards among the muscles. 



The structure of the pectoral fin is very simple. Articulated to 

 the posterior border of the scapula and coracoid are four dice-box- 

 shaped bones, the proximal pterygiophores or radials (PTG. 1), 

 followed by a row 

 of small nodules of 

 cartilage (ptg. 2) 

 representing distal 

 pterygiophores. 

 The main body of 

 the finis supported 

 by dermal fin-rays, 

 which resemble 

 those of the 

 median fins, and 

 have their forked 

 ends seated upon 

 the distal ptery- 



tiophores : the 

 rst ray, however, 

 is larger than the 

 rest, and articu- 

 lates directly with 

 the scapula. 



There is no pel- 

 vic girdle, its 



PTG.l 



FIG. 874. Salmo fario. Left half of shoulder-girdle and 

 pectoral fin, from the inner surface. CL. clavicle ; COR. 

 coracoid ; D.F.R. dermal fin-rays ; MS. COR. mesocoracoid 

 P.CL., P.CL\ post-clavicles; PTG.l, proximal, and ptg. 2 

 distal pterygiophores ; P. TM. post-temporal ; S. CL. supra- 

 clavicle ; SCF. scapula. 



place being taken by a large, flat triangular bone, the basipterygium 

 (Fig. 875, B. PTG.), probably representing fused proximal pterygio- 

 phores : to its posterior border are attached three partly ossified 

 nodules, the distal pterygiophores (PTG.), and with these the 

 dermal fin-rays are articulated. The adipose lobe of the pelvic 

 fin is supported by a small scale-like bone. 



The muscles of the trunk and tail are arranged, as in the Dog-fish, 

 in zigzag myomeres : there are small muscles for the fins, and the 

 head has a complex musculature for the movement of the jaws, 

 hyoid, operculum, and branchial arches. 



The coelome is divisible into a large abdomen (Fig. 876) con- 

 taining the chief viscera, and a small pericardial cavity, situated 

 below the branchial arches, and containing the heart. 



