i68 ANIMAL LIFE PAST AND PRESENT. 



a smaller bone known as the coracoid-bone, which 

 forms part of the cavity for the head of the arm-bone, 

 precisely as in the reptilian shoulder-girdle represented 

 in Fig. 50. Further, in advance of the coracoid-bone, 

 and below the shoulder-blade, the Monotremes have a 

 third bone in the part of the shoulder-girdle correspond- 

 ing to the one marked p. cor. in Fig. 50, which should 

 likewise be known as the precoracoid, although it is 

 often termed the epicoracoid. We thus see that in 

 this part of their shoulder- girdle the Monotremes are 

 totally unlike all other mammals, and resemble that 

 extinct group of Eeptiles of which this part of the 

 skeleton is shown in the woodcut. A further resem- 

 blance to Eeptiles is, however, indicated by the presence 

 of a peculiar T-shaped bone lying on the front of the 

 chest between the collar-bones or clavicles, and hence 

 termed the interclavicle ; this interclavicle occurring 

 not only in that group of Eeptiles to which the figured 

 shoulder-girdle belongs, but also in Lizards, Fish-Lizards, 

 and other types. 



With regard to the shoulder-girdle represented in 

 Fig. 50, it should be observed that in all existing 

 Eeptiles the precoracoid (p. cor.) becomes united either 

 with the coracoid or the shoulder-blade, so that in 

 many cases it cannot even be recognised as a distinct 

 element at all. The same is the case with all extinct 

 groups of Eeptiles, with the sole exception of one very 

 remarkable group found in the early Secondary rocks 

 of South Africa, India, North America, and parts of 

 Eussia, and known, from their peculiar types of teeth, 



