90 COMPARATIVE ANATOMY. 



As in the rest of the skeleton, cartilage plays the most im- 

 portant part in the pectoral arch of Amphibia (see Fig. 74), while 

 in all the higher Vertebrates the cartilage is almost entirely 

 replaced by bone (scapula, coracoid, and clavicle). Unossified 

 spaces are often left in the coracoid, giving rise. to fenestras closed 

 over by fibrous membrane (Lizards) (Fig. 39, a, b, c). 



In Lizards, a cross or T-shaped parosteal bony plate, the inter- 

 clavicle, lies in a groove on the under side of the sternum in the 

 middle line. In Crocodiles a slender rod-like interclavicle is also 

 present, and has the same relations, though clavicles are wanting. 1 



FIG. 74. PECTORAL ARCH AND STERNUM OF Bomlinator igneus. 



St, sternum, with its two processes, a, a 1 : S, scapula ; SS, suprascapula, in situ on 

 the left side, spread out horizontally on the right ; Co, coracoid ; Co 1 , epicoracoid, 

 which on both sides overlaps the anterior part of the sternum ; Cl, precoraeoid ; 

 Cl l , bony clavicle ; Fe, fenestra between clavicle and coracoid ; G, glenoid 

 cavity. 



The presence of a pectoral arch in numerous footless Reptiles 

 (some Skinks, Amphisbaenians) indicates that they formerly pos- 

 sessed extremities ; rudiments of the latter may even be seen in the 

 embryo, though they disappear entirely later on (Anguis fragilis). 



Birds. In Birds, the scapula consists of a thin and narrow 

 plate of bone, often extending far backwards, the strong coracoid 

 being bent at a sharp angle with it in all Carinate Birds (Fig. 36, 

 S and Co). The lower end of the latter is firmly articulated in 

 a groove on the anterior edge of the sternum. 



In almost all Flying Birds the clavicle is well developed, and 

 becomes united with its fellow to form a furcula 2 (comp. Fig. 36, 



1 It is doubtful whether the three anterior elements of the plastron of Chelonians 

 correspond to clavicles and interclavicle, as supposed by Parker and Huxley. 



2 The median plate often present at the point of junction of the clavicle is some- 

 times described as an interclavicle, but its late appearance in the embryo seems to 

 negative this view. 



