88 THE MAMMALIA. 



all these possibilities into consideration without 

 gaining anything for the point in question. 



It is, however, a different matter with the 

 following characteristics. The Cloacal animals aiv 

 the only Mammals where the collar-bones have 

 become united into one piece by means of the 

 breast-bone, as in the case of the well-known fork- 

 bone in birds. In all other Mammals, including 

 Man, we find, in place of this free os cora<-<>i<l<'iun, 

 a short hook, the crow-beak bone. This circum- 

 stance, in and of itself, might lead to the supposition 

 that the Duck-moles were more fully developed than 

 the other Mammals. However, from the course of 

 the development as a whole, it is obvious that 

 this portion of the shoulder of the higher Mam- 

 mals must be regarded as a case of reversion. 

 Another peculiarity of the skeleton of the Mono- 

 trema is a couple of bones which, turned towards 

 the front, rise above the pubic bones at the 

 abdomen side of the pelvis. These bones exist also 

 in the Marsupials. As, however, we are uncertain 

 about their origin and significance even in the case 

 of the Marsupials, nothing much can be made out 

 of the agreement. Still, the occurrence of these 

 bones in the two classes points to a close relation- 

 ship. 



