tJje Teeth of graminivorous Quadrupeds. 253 
of knowing how far this general rule applied to quadrupeds at 
large, and, if it did not, in what animals the teeth were diffe- 
rently 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 two 
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. This will 
be better understood by referring to the figures, (Tab. XX.) 
which represent the appearance of the enamel in different sec- 
tions of the tooth. 
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 cannot af- 
terwards be readily removed ; so that the grinding surface will 
be always kept irregular, and in a still greater degree than in 
any of the other teeth which have been described. The parti- 
cular form of this tooth is represented in Tab. XXI. 
It is highly probable that there are many other varieties in 
the structure of the grinding teeth of quadrupeds, but these will 
jmdccxcix. L 1 
