122 ON THE ARTICULATION OF THE VERTEBRATE JAW. [Feb. 6, 



which have flat-topped molar teeth, for crushing or grinding food, 

 i-equire a jaw mechanism which will allow them to separate the 

 lines of their teeth slightly without throwing them greatly out of 

 the parallel, and then to bring the opposed surfaces of these teeth 

 together simultaneously ; and that this requirement is met by 



Text-fig. 47. 



Skull of a Batagiir Toitoise. 



articulating the lower jaw with the skull on a plane either above 

 or below that of the opposed surfaces of the teeth. Farther, 1 

 think that such an arrangement is favouiuble, if not necessaiy, 

 to horizontal movements of the teeth over one another. 



Some Inferences. 



So far I have been dealing with the subject from a purely 

 mechanical point of view. It is, however, extremely tempting to 

 speculate upon its evolutionary aspects also. I append a few 

 suggestions ; but they are, of course, purely tentative. 



I regard jaws of type 1 as the original type, and those of type 2 

 as a later improvement. I think there is ground for this view 

 not only in the fact that type 1 is simpler and the form found in 

 the lower vei^tebrates, but also in the development of the human 

 jaw. At birth the angle is slight, the condyle being at a low 

 level. As the molar teeth develop from before backwards the 

 angle approaches a right angle, the condyle rising. Also as an 

 abnormality teeth sometimes appear which continue the series of 

 teeth backwai'ds up the ascending ramus of the jaw. 



If jaws of type 1 preceded jaws of type 2, the first terrestrial 

 vertebrates wei'e probably animal-food eaters. They probably left 

 the watei- to prey upon the invertebrates, which were flourishing 

 on the land plants, and in course of time they learnt to eat the 

 more succulent fruits. Some modern lizards, which in general 

 appearance and usual habits are animal-food eatersj will vary 

 their diet by eating a little ripe fruit occasionally. 



From soft fruits some of the reptiles passed on to fleshy leaves, 

 but it is doubtful whether they got much further. Owing to the 

 big quadrate bone they could not develop jaws of type 2, so 



