MAMMALIA 



69 



windpipe projects into the back of the nasal passages, so that the 

 feeding and breathing tracts are kept separate. 



There are also two well-marked features of the skeleton, one 

 concerning the lower jaw and the other the pelvis. The hinder 

 corners or angles of the former are bent (inflected) inwards 

 (fig. 37), while the latter possesses two extra bones (epipubic 

 bones) which are at most represented by 

 vestiges in members of the Eutheria. 



There is only one order, i.e.: 



13. Pouched Mammals (Marsupials), of which all 

 the existing types are limited to Australia and 

 its adjacent islands, with a few exceptions 

 found in America. — Exs. : Kangaroo, Wom- 

 bat, American Opossums. 



Fig, 37, — Lower Jaw of a Wombat 

 from behind. Note inwardly bent 

 angles. 



C. Prototheria. — A very small number of Mammals con- 

 stitute this group, and they are marked off from the higher forms 

 by the possession of very remarkable characters. Agreeing with 

 Marsupials in the characters of the brain, which is, however, 

 of a still lower type, and the presence of epipubic bones, they 

 differ from them and all other Mammals in the fact that they 

 lay tough-shelled eggs. The young, when hatched, are nourished 

 by milk, as in other cases, but the milk-glands possess no teats, 

 simply opening upon a bare patch of skin. The Prototheria are 

 also distinguished by the comparatively low 

 temperature of the blood, the peculiar structure 

 of the shoulder- girdles, and the fact that the 

 intestine ends by opening into a chamber, the 

 cloaca, which also receives the products of 

 nitrogenous waste. As to the shoulder-girdle, 

 it was mentioned in describing the human 

 scapula (p. 30) that a coracoid process is pre- 

 sent on that bone, and that the process in 

 question must be regarded as the vestige of 

 what in ancestral forms was a distinct bone. 

 Prototheria possess a distinct coracoid bone 

 (fig. 38), and also certain other bony elements in the shoulder- 

 girdle, approximating in this respect to what is found in lizards 

 among the Reptiles. It may indeed be noted generally that the 

 peculiar characteristics of these lowest Mammals point to reptilian 

 affinities. 



Fig. 38. — Sternum and Shoulder- 

 girdles of Duck-Mole, from below 

 cr, Coracoid bone, just above 

 which is the cup (glenoid cavity) 

 for attachment of upper-arm 

 bone. 



