THE DESCENT OF THE PRIMATES 29 



even Osborn accepts the term Sauromammalia in 

 exchange for Huxley's Hypotheria, thereby im- 

 plying that certain distinctive characters of those 

 Proto-mammalia, which can hardly have been 

 other than such as belong to their reproductive 

 and developmental arrangements, were distinctly 

 Saurian. 



With respect to this important question I feel 

 inclined to side rather with Huxley — whose 

 brilliant vindication of the amphibian character 

 of his Hypotheria ^ is a perfect model of morpho- 

 locjical arsfument — than with the views last- 

 mentioned, and I will attempt to explain my 

 reasons for so doing. The differences between 

 Eeptilia and Amphibia, as exhibited by living 

 representatives of the two classes, are nowhere so 

 obvious as in the fact that the former possess an 

 important transitory embryonic structure known 

 as the amnion, whereas the latter do not. In 

 going back into the Mesozoic and Palseozoic 

 periods we are led to presume that such extinct 

 orders as the Pterosauria, Dinosauria, etc., shared 

 these developmental features with the modern 

 reptiles. In fact the large eggs which some of 

 the latter are supposed to have laid are preserved 

 in certain Museums. But when we come to yet 



1 Proc. Zoul. Society, 1880, p. 649. 



