THE PHYLOGENESIS OF VERTEBRATES 33 



jtegocephali and the mammals requires some intermediate 

 link, and thus the addition of the theromorphs in this place 

 seems rather a completion than an opposition to the theory 

 of Stegocephalan ancestry. 



The only living reptiles associated with the same branch as 

 the theromorphs are the turtles (Chelonia), which, although 

 highly specialized in the matter of trunk skeleton, are of the 

 greatest value in regard to their soft parts, which are un- 

 doubtedly similar to those of the extinct members of the 

 branch, and are thus the best living representatives of 

 the important stage between amphibians and the early 

 mammals. 



The earliest mammalian remains are contemporary with 

 those of the theromorphs, and are those of small forms, like the 

 most mammalian of the reptilian remains. These are ap- 

 parently nearly related to the monotremes, the lowest living 

 mammals, which are represented by two forms occurring in 

 Australia and New Zealand, the Duck-bill Platypus (Ornith- 

 orhynchus) and the spiny ant-eater (Echidna). The latter 

 has no connection with the true ant-eaters (Myrmecophagida) 

 of South America, which are placental mammals. The mono- 

 tremes are strongly reptilian in certain skeletal features; like 

 true reptiles and unlike all other mammals, they possess a 

 single terminal orifice, that of a common cloaca, into which 

 open the alimentary canal, the ureters and the genital 

 ducts; and they actually lay eggs, that is, very immature em- 

 bryos, surrounded by a thin, cornified shell. The mammary 

 glands, one of the essential characteristics of the class of mam- 

 mals, are seen here in a very simple condition. They consist of 

 two lateral groups of integumental glands, apparently of the 

 tubular type, which open separately in the bottom of an oval 

 depression, the mammary pocket. There are no teats, and 

 the young obtain the secretion either directly from the de- 

 pressions or by sucking at the hair in this region. 



The next group above the monotremes are the marsupials, 

 with the exception of the opossum also confined to the Aus- 

 tralian region. As in the previous group, the young are born 



