8 REPTILES 



the Tertiary period and in the existing world being compara- 

 tively degenerate types." 



A few of the distinctive features of the reptilian class have 

 been already mentioned ; these and others may now be some- 

 what more fully noticed, especially when they display marked 

 adaptive or evolutionary modifications. First, with regard to 

 the single occipital condyle by means of which the skull is 

 articulated to the atlas, or first, vertebra in all existing reptiles 

 and probably also in all extinct ones with the possible exception 

 of some of the anomodonts. In its most typical form, as 

 exemplified by crocodiles and alligators, this condyle consists of 

 a single knob formed exclusively by the basioccipital bone of 

 the skull ; it is then known as the typical monocondylic form. 

 In many reptiles, such as iguanas, pythons, and turtles, lateral 

 elements are developed to a larger or smaller extent from the 

 exoccipital bones, and the structure then becomes of the tri- 

 partite monocondylic type. In some tortoises (Testudo), and 

 more especially in the anomodonts (such as Dicynodon and 

 Cynognathus) the lateral elements tend to develop at the ex- 

 pense of the median basioccipital element, which becomes 

 greatly reduced, and we thus have the transitional dicondylic 

 type, which, it is noteworthy, also exists in certain mammals. 

 Finally, the total disappearance of the basioccipital element 

 would result in the typical dicondylic form, or, in other words, in 

 the double condyles of ordinary mammals. Such (to some- 

 what anticipate matters) appears to have been the mode in 

 which the double occipital condyles of mammals have been 

 evolved from the single reptilian condyle. Here it may be 

 mentioned that amphibians also possess double condyles, and 

 it was accordingly at one time supposed that mammals took 

 their origin from amphibians rather than from reptiles, but 

 there are structural features which militate strongly against such 

 a view. The single monocondylic type has, on the other hand, 

 persisted in the evolution of reptiles to give origin to birds. 



The next most important feature in the skeleton of reptiles 

 is the fact that, as in birds, the mandible, or lower jaw, consists 

 of a number of separate bony elements, and articulates with the 

 squamosal region of the cranium, or skull proper, by means of 

 a quadrate bone. Much discussion has taken place as regards 

 the fate of the quadrate bone when the theromorph reptiles 



