THE SKELETAL SYSTEM 



231 



of the upper jaw and the dentary or mandibula of the lower. These 

 membrane bones, however, do not extend as far as the articulation of the 

 jaw which, as in amphibia, is between the quadrate of the upper jaw and 

 the articulare of the lower. 



In mammals a new articulation of the jaw is formed between two 

 dermal bones, the dentary of the lower jaw and the squamosal of the 

 brain-case. The evidence for this change in the articulation of the jaw is 

 too strong to permit of any reasonable doubt. How such a change could 



OUTER ASPECT. 



Fig. 188. — Jaws of Tertiary reptiles viewed from their inner and outer aspects. 

 Dimetrodon belongs to the lower Tertiary, while Scymnosuchus and Cynognathus 

 belong respectively to the middle and upper Tertiary. A notable increase in the size 

 of the dentary and relative diminution in the size of the articular element is seen. Such 

 changes suggest how in the ancestors of mammals the articulation of the lower jaw 

 shifted from the articulare to the dentary. Thus the articular was set free to become 

 the malleus of the ear. By similar changes in the upper jaw the squamosal bone replaced 

 the quadrate, which then became the incus. The teeth of the three jaws also show a 

 change in the dentition in the direction of that of mammals. (Redrawn after D. M. S. 

 Watson.) 



occur while the jaw was still functioning has been one of the vexed ques- 

 tions in vertebrate morphology. Thanks, however, to recent discoveries 

 of fossil reptiles which bridge the gulf between reptiles and mammals, the 

 way in which the shift was effected is now fairly clear. (Fig. 188) 



D. M. S. Watson and others have found that in Tertiary reptiles 

 changes in the articulation of the jaw occurred which involved reduction 

 of the articulare element and a corresponding enlargement of the dentary. 

 The relation of the two bones is such that the lower jaw at one time was 

 probably hinged by both. The enlargement of the dentary and the 

 diminution of the articulare set free the latter to pass into the tympanic 



