126 THE OSTEOLOGY OF THE REPTILES 



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 clei- 

 thrum on each side, all live of dermal origin, together composing the 

 secondary or clavicular girdle; and three bones on each side, the 

 scapula and two coracoids,i 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 rep- 

 tiles 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 am- 

 phibians, 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 re- 

 duced. The girdle in the adult land forms, of which Eryops (Fig. 108) 

 and Cacops (Fig. 96 a) may be taken as types, is almost indistinguish- 

 able 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 Stego- 

 cephaHa (Figs. 96 a, 108) was doubtfully ever functional in reptiles, 



1 [According to Watson, the coracoid originally was a single piece which never be- 

 came subdivided in the amphibians, cotylosaurs, or ordinary reptiles, the subdivision 

 occurring only in the Theromorpha, Therapsida, and mammals. — Ed.] 



