The scapular arch also is more complicated and more important in the lower than in the higher vertebrata. It may include three bones on each side named coracoid, precoracoid, and scapula. But in most vertebrates the coracoid and precoracoid appear never to have been segmented so as to be separated from each other; and it is only among extinct types of reptiles, which appear to approximate to the Monotreme mammals, that separate precoracoid bones are found; though among most mammals, probably, there are stages of early development in which precoracoids are represented by small cartilages, though few mammals except Edentata like the Sloths and Ant-eaters, retain even the coracoids as distinct bones. Therefore, excepting the Edentata and the Monotremes, the distinctive feature of the mammalian shoulder-girdle appears to be that the limbs are supported by the shoulder-blades, termed the scapulæ. Among reptiles there are several distinct types of shoulder-girdle. Chelonians possess a pair of bones termed coracoids which have no connexion with a sternum; and their scapulæ are formed of two widely divergent bars, divided by a deeper notch than is found in any fossil reptiles. Among Lizards both scapula and coracoid are widely expanded, and the coracoid is always attached to the sternum. Chameleons have the blade of the scapula long and slender, but the coracoid is always as broad as it is long. Crocodiles have the bone more elongated, so that it has somewhat the aspect of a very strong first sternal rib when seen on the ventral face of the animal. The bone is perforated by a foramen, which would probably lie in the line of separation from the precoracoid if any such separation had ever taken place. The scapula, or shoulder-blade, of Crocodiles is a similar flat bone, very much shorter than the scapula of a Chameleon, and more like that of the New Zealand Hatteria. Thus there is very little in common between the several reptilian types of shoulder-girdle. [Illustration: FIG. 38. COMPARISON OF SCAPULA AND CORACOID IN THREE PTERODACTYLES AND A BIRD]