OK THE BONES OF THE HIPPOPOTAMUS. 379 



itself to the anterior and posterior so, that, in the skeleton, two-thirds 

 of the OS tympani are surrounded by a vacant space. 



The shape of the lower jaw is also very remarkable ; its two almost 

 parellel branches (a b, fig. 4, plate 31), instead of forming a con- 

 tracted point at their junction, expand themselves at that point into 

 a space almost square, on the anterior edge of which, c d, the incisores 

 are implanted on a right line, while its angles e, project obliquely in 

 front to sustain the canine teeth. 



Viewed on the side, the branch of the jaw is remarkable for the 

 extremely prominent angle (/, plate 31, fig. 1), iri the form of a half 

 crescent, which it makes below, and which is determined by a large 

 semicircular slant, g. The posterior edge of the ascending branch is 

 remarkably thick. 



The condyle forms an irregular cylinder, and descends from- the 

 outside to the inside. The holes for the issue of the nerve are two 

 or three in number; they are below the first molar, and somewhat 

 forward. 



II.— The Teeth. 



There is no animal which requires to be more studied at different 

 ages, in order to acquire a perfect knowledge of its molar teeth, than 

 the hippopotamus; they change their shape, tlieir number, and their 

 position. The definite number is six on each side, above and below — 

 twenty-four in all ; and, as in the horse, there are three in front which 

 are renewed, while the three posterior are not renewed. There is 

 moreover, as is in the horse, a tooth in front (I, plate 31, fig. 3), which 

 falls W'ithout being replaced. 



Hence there are four sucking molars, three that replace others, 

 and three back molars. 



The first three sucking grinders and the three that replace the others 

 have a peculiar shape, being conical and much more simple than that 

 of the back grinders. 



The fourth sucking grinder, on the contrary, resembles the ■ back 

 grinders in its complicated shape. It is replaced by a simple grinder 

 (n, ib.') ; but as at the same time the last back molar, t, emerges from 

 the jaw, the number of complex grinders always remains the same, 

 viz. three. 



It is a general rule that the sucking grinders of all animals partake 

 of the complex form of the back grinders in a greater degree than the 

 replacing gi-inders ; and the reason for this is plain, viz. that the sucking 

 grinders might in some measure fulfil the functions of the back grinders, 

 which have not as yet entirely emerged. 



This shape, to which I have given the name complex, consists more 

 especially in the hippopotamus in four conical hills, ranged two and 

 two, so that one pair may be before the other transversely. Each of 

 these hills is intersected, on the surfaces not contiguous to each other, 

 by two deep longitudinal furrows ; so that the crown of the tooth, when 

 it begins to be worn, presents the figure of a double trefoil for each 

 pair of hills. When detrition has descended as far as the point where 

 the hills become united, a quadrilobed figure is formed for each pair. 



