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



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

 require 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 Hat ay 117' Tortoise. 



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

 or below that of the opposed surfaces of the teeth. Further, I 

 think that such an ari'angement is favourable, if not necessaiy, 

 to hoiizontal 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 f oi-m found in 

 the lower vertebrates, 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 fi'om before backwards the 

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

 abnormality teeth sometimes appear which continue the series of 

 teeth backwards up the ascending ramus of the jaw. 



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

 vertebrates were probably animal-food eaters. They probably left 

 the water 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 eatei'S, 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 doubtfid whether they got much fiu'ther. Owing to the 

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



