xii. 8 SHOULDER GIRDLE 309 



medial and lateral braces (adductors and abductors) and the muscles 

 drawing the leg backwards and forwards are posterior and anterior 

 (retractor and protractor) braces. For the attachment of these muscles 

 proximally the pectoral and pelvic girdles, small in fishes, become 

 expanded into plates (Figs. 180 to 182), and these are divided into 

 a number of characteristic pieces, though the mechanical reason for 

 the division is not clear. 



8. Shoulder girdle of Amphibia 



The earliest labyrinthodonts (e.g. *Eogyrinus) inherited a shoulder 

 girdle almost exactly like that of their osteolepid ancestors except that 

 a new dermal element, the interclavicle, was added to the ventral 

 surface. Although the presence of a sternum has never actually been 

 recorded, it is generally assumed that a cartilaginous structure of this 

 type was present between the hindermost margins of the epicoracoid 

 cartilages. As in gnathostomes generally (except elasmobranchs) the 

 shoulder girdle was a dual structure consisting of (a) a primary or 

 endochondral component evolved from the basal fin elements of the 

 ancestral fish form and serving to provide an articulatory surface for 

 the limb as well as points of attachment for the limb musculature, and 

 (b) a dermal ring of bony elements (skin scales) which had sunk 

 inwards and applied themselves to the ventro-anterior surfaces of the 

 endochondral girdle which, consequently, they braced and supported. 



The endochondral girdle consisted of two half rings, which over- 

 lapped in the ventral midline. Each half was a single unit but, by 

 topographical comparison with girdles of later tetrapods, it is often 

 arbitrarily divided into two regions, a dorsal scapula and a ventral 

 coracoid. Between these two regions a screw-shaped glenoid received 

 the humerus. The one endochondral ossification is usually homo- 

 logized with the scapula of amniotes (Watson, 191 7). Later forms 

 (e.g. * Seymonria, *Diadectes) possessed a second bony element which 

 is generally interpreted as a precoracoid. The endochondral girdle was 

 small in the earliest amphibia (e.g. *Eogyrinus). In later genera its size 

 progressively increased, presumably to withstand the greater thrust 

 transmitted by the larger limbs of these forms and to provide attach- 

 ment for the increased mass of brachial musculature. 



The dermal girdle consisted, typically, of paired cleithra, clavicles, 

 and interclavicle. The latter, a new element, lay between and often 

 beneath the clavicles and, together with the sternum, probably formed 

 a locking mechanism preventing the complete separation of the epi- 

 coracoid cartilages. In the earliest rhachitomes (e.g. *Eogyrinus) the 



