the epicoracoid, are yet doubtful, and will be discussed later. There is also a median, unpaired bone in these mammals, the interclavicle, unknown in other mammals.
Primitively (Figs. 95, 96), that is, in the oldest known reptiles, the pectoral girdle is composed of eleven separate and distinct bones, at least in early life: the median interclavicle and a clavicle and cleithrum on each side, all five of dermal origin, together composing the secondary or clavicular girdle; and three bones on each side, the scapula and two coracoids,[1] all of endoskeletal origin, composing the primary or scapular girdle.
The cleithrum (Fig. 95), a relic from the fishes, disappeared in Triassic times, after long existence as a mere vestige. The posterior of the two coracoids also disappeared in late Triassic times, in reptiles at least, though a vestige may possibly be present in our own shoulder girdle. The scapula, clavicles, and anterior one of the two coracoids, the so-called procoracoid, are still present in most reptiles; in snakes only are they wholly absent, though much reduced and non-functional in some lizards.
Clavicular Girdle
The clavicular girdle is variable among the temnospondyl amphibians, dependent, as in reptiles and higher vertebrates, upon the habits of the animals. In the aquatic types of all Stegocephalia the clavicles and interclavicles are rugose [on the ventral side], heavy and broad, forming more or less of a pectoral buckler—a peculiar adaptation to their water habits, perhaps in a measure analogous to the plastron of the turtles or the extraordinary development of the coracoids in the plesiosaurs. In such forms also, the cleithrum is reduced. The girdle in the adult land forms, of which Eryops (Fig. 108) and Cacops (Fig. 96 a) may be taken as types, is almost indistinguishable from that of their contemporary cotylosaurs, except that the cleithrum is larger and the interclavicle less elongate. They are smooth throughout in Cacops, the more terrestrial form.
Cleithrum. The cleithrum so generally characteristic of the Stegocephalia (Figs. 96 a, 108) was doubtfully ever functional in reptiles,
- ↑ [According to Watson, the coracoid originally was a single piece which never became subdivided in the amphibians, cotylosaurs, or ordinary reptiles, the subdivision occurring only in the Theromorpha, Therapsida, and mammals.—Ed.]
