XIII 



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 fin is supported 

 by dermal fin-rays, 

 which resemble 

 those of the 

 median fins, and 

 have their forked 

 ends seated upon 

 the distal ptery- 

 giophores : the 

 first ray, however, 

 is larger than the 

 rest, and articu- 

 lates directly with 

 the scapula. 



There is no pel- 

 vic girdle, its 

 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 ccelome 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. 



FIG 



DFR. 



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

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

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

 P.CL., P.CL\ post-clavicles; PTG.l, proximal. &n&ptg. - 

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

 clavicle ; SCP. scapula. 



