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Show 122 ON THE ARTICULATION OF THE VERTEBRATE 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 Batagur 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 arrangement is favourable, if not necessary, 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 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 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 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 eaters, 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 |