XIII 



PHYLUM CHORDATA 



209 



glenoid facets, and developed partly as a replacing, partly as 

 an investing bone ; a coracoid (COR.), situated ventrally to the 

 glenoid facet, and a meso-coracoid (MS. COR.), situated above tbe 

 coracoid and anterior to the scapula. Externally to these is found 

 a very large investing bone, the clavicle (CL.}, extending down- 

 wards 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 



COR. 



PTG.I 





FIG. 8<53. Salmo fario. Left half of shoulder-girdle and pectoral fin, from the mier surface. 

 CL. clavicle ; COR. coracoid ; D.F. R. dermal fin-rays ; WIS. COR. mesocoracoid ; P.CL., 

 P. CL'. post-clavicles; PTG.I, proximal, and pty. %, distal pterygiophores ; P. TM. post- 

 temporal ; CL. supraclavicle ; SCP. scapula. 



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) repre- 

 senting 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 pterygio- 

 phores : the first ray, however, is larger than the rest, and 

 articulates directly with the scapula. 



There is no pelvic girdle, its place being taken by a large, flat 

 triangular bone, the basipterygium (Fig. 864, B. PTG.), probably 

 representing fused proximal pterygiophores : to its posterior border 



