Of the two coracoid bones in the original pectoral girdle the posterior one began to disappear early and is entirely lost in all reptiles that lived later than Triassic times, though it still persists in the lowest mammals, as we have seen. In most later reptiles the remaining coracoid has become less firmly attached to the scapula than it was in the older reptiles. It usually has a small foramen piercing it near the middle of the upper border or end, the supracoracoid foramen. The clavicle, while more constant among reptiles than among mammals, has been lost in some, the Crocodilia, for instance, as also the dinosaurs and pterodactyls. The interclavicle is more constant in reptiles, a more or less =T=-shaped bone underlying the coracoids where they join, or the breast bone; but there were some reptiles that lost it, the dinosaurs and pterodactyls, for instance. In the turtles both the clavicles and the interclavicle form a part of the under shell or plastron. [Illustration: FIG. 19.—Scapula (_sc_), coracoid (_cor_), and metacoracoid (_mcor_) of _Dimetrodon_] The cleithrum is known in only a few of the old reptiles; it is a more or less slender bone which lies along the upper front margin of the scapula, articulating at its lower end with the upper end of the clavicle on each side. The breast bone or sternum, while not properly a part of the pectoral girdle, may be mentioned here. In reptiles it is rarely well developed or even ossified, the flying reptiles known as the pterodactyls being the most notable exceptions. It was a comparatively late development in this class, the earliest ones not possessing it even in a cartilaginous condition. It was doubtless evolved from the more or less numerous and slender ossifications on the under side of the body called ventral or abdominal ribs, after the coracoids had become reduced and more slender. Whenever it is present the coracoid articulates with it on each side in front. In most lizards it remains as a cartilage throughout life. [Illustration: FIG. 20.—Clavicles and interclavicle of _Ophiacodon_, a theromorph reptile from the Permocarboniferous of New Mexico.] ANTERIOR EXTREMITY