ONTOGENY AND PHYLOGENY OF THE STERNUM 81 



their reptilian affinities. It is thought that this makes a strong 

 case for the homology of the sternum in the Amniota and its 

 origin in connection with the coracoids ; and since this same struc- 

 ture is undoubtedly coracoidal in the Ichthyopsida, the two, 

 Amniote and Ichthyopsidan sternum, are homologous. 



As this work of Broom's is of considerable importance to our 

 argument, it may be stated that Broom in his later papers veri- 

 fied his first discoveries in representatives of all groups of mar- 

 supials and made graphical reconstructions of the parts. His work 

 has been checked up by Watson ('17), who made wax models 

 of the girdles and sternum, and completely confirmed Broom's 

 results. 



7. Aquatic mammals 



Among the aquatic mammals there are several interesting 

 sterna. In the adult Manatus americanus (fig. 40) the sternum 

 is moderately large and is typically divided into the usual 

 three parts. This animal has seventeen pairs of ribs, but only 

 three come anywhere near the sternum, and Parker ('68) says that 

 only the second pair of ribs reaches it. The first and third pairs 

 are connected with it by ligament only. Here in this adult 

 mammal is a stage comparable to the embryo of the pig, man, 

 etc., where ribs either do not reach the sternum or are connected 

 with it by fibrous tissue. It is difficult to believe that this entire 

 sternum was derived from the costal ends of three pairs of ribs, 

 of which only one pair even approaches it. 



According to W. K. Parker ('68) the sternum of the dolphin 

 embryo (fig. 41) has reached its highest development in aquatic 

 mammals. In the stage figured the sternal bands have fused, 

 leaving a prominent longitudinal groove to mark the line of 

 fusion, several centers of ossification are present, and in the pre- 

 sternum is an oval fontanelle such as is common in the lizards. 

 In fact, this entire structure is very reptilian in character, as is 

 seen by a comparison with Trachydosaurus (fig. 28) , even to the 

 number of ribs. 



In orders of mammals higher than those already mentioned 

 the literature is scanty. Practically all papers based upon the 



