526 PHILOSOPHICAL TRANSACTIONS. [ANNO 1790. 



in their shape, forming irregular cylindrical tubes, the central part of which is filled 

 up by the projecting membranes from the gums, that are to be changed tor bone. 

 The division of the tooth into 2 parts, is very distinct in the shedding teeth, but 

 not in the 2d set or permanent teeth. These 2 portions of bone in the middle of 

 the tooth have frequently a hole in them, probably the passage of a blood-vessel, 

 never completely filled up, and the food getting into it, as the tooth is worn down, 

 considerably increases its size. Besides which, there is a portion of bony substance 

 surrounding a great part of the outside of the tooth. 



In the cow's grinding teeth, there are 2 portions of bony substance in the 

 middle of the tooth, as in the horse, in shape of crescents, and a very small por- 

 tion in the hollows on the outside of the circumference of the tooth; but none on 

 the projecting parts. In the grinding teeth of the sheep, the middle portions of 

 bone are similar to those of the cow, but on a much smaller scale; there is no 

 portion of bone on the outside of the tooth. It is not to be wondered at, that there 

 is so great a variety in the grinding surfaces of the teeth of different genera of 

 graminivorous quadrupeds, each no doubt adapted to the kind of food they are in 

 a state of nature destined to live on, since there is even a variation between the 

 teeth of the African and Asiatic elephants. In the African elephant, the processes 

 of which the tooth is composed are not flattened ovals, as they have been described 

 in the Asiatic, but are in the form of an oblong square or parallelopipedon, so 

 that, in the middle line of the tooth, the processes are in contact with each other, 

 though at no other part; by this means, the middle line of the tooth is the 

 hardest; the whole surface therefore does not wear regularly, as in the Asiatic 

 elephant, but with a ridge in the middle. 



Having, by the foregoing observations, established a well marked characteristic dis- 

 tinction between the teeth of truly carnivorous and truly graminivorous quadrupeds, I 

 was desirous of knowing how far this general rule applied to quadrupeds at large, and 

 if it did not, in what animals the teeth were differently formed. The teeth of the 

 hippopotamus and rhinoceros are found to differ in their structure from those above 

 described, partaking in some measure of the properties of both, and forming 2 very 

 curious links in the chain of regular gradation between the one and the other. 

 The grinding teeth of the hippopotamus are made up of the substance of the tooth 

 and enamel only, having no portion of bone mixed with the other parts; but, what 

 is I believe peculiar to them, the enamel pervades the substance of the tooth to a 

 considerable depth, so as to be intermixed with it. The grinding teeth of the 

 rhinoceros have a peculiarity of a very different kind: they also are only composed 

 of the substance of the tooth and enamel; but the tooth is so formed as nearly to 

 surround a middle space, which, were it filled up with bone, would make a truly 

 graminivorous tooth, not unlike those above described. This middle space is left 

 open, and becomes filled up with the masticated food, which falls into it, and can- 

 not afterwards be readily removed; so that the grinding surface will be always kept 



