40—6 INDIAN TERTIARY AND POST-TERTIARY VERTEBRATA. 
cliaracters being most marked in tlie specimen represented in figs. 2, 2a, 2b, wliere 
they are accompanied by a lower occipital surface {fig. 2b). The transition from the 
typical skulls to this specimen is, however, so gradual that, in the absence of other 
differences, it appears impossible to draw any specific distinction on this score ; but 
the specimens with more prominent orbits undoubtedly exhibit a marked step from 
the type in the direction of the Narbada and existing hippopotami. 
Mandible . — Several specimens of the mandible are represented in plates LX., 
LXI. of the “ F.A.S.” : those in figs. 3 and 6 of the former showing the milk- 
dentition : a specimen of the anterior portion of the symphysis represented in plate 
LXI., fig. 7, is refigured on a smaller scale in the woodcut on page 38. The three 
incisors are sub-equal in size ; the median pair being rather larger than either of the 
others : at the free edge of the mandible' the six teeth form an even line, but the 
lower part of the alveolus of i. 2 is placed above the level of the other two alveoli.^ 
These teeth are jfiaced more obliquely than in II. amphibius, and the abraded surface 
is more confined to their extremities, indicating affinity to the 2 :)igs ; the canines are 
relatively shorter and larger ; and pm. 1 is retained longer. The descending plate 
at the angle^ is considerably larger and deeper than in II. amphibius, and its anterior 
border is blunt and rounded, instead of curved and pointed anteriorly. The mandible 
of the Siwalik species is also distinguished by its longer symphysis, of which the 
superior surface is more deeply channelled. The line of the lower cheek-teeth and 
the horizontal ramus of the mandible, is markedly concave externally, instead of 
nearly straight.^ It does not appear that the lower cheek-teeth of the two forms 
jwesent any characteristic points of distinction. The following table exhibits the 
dimensions of the mandible® of II. sivalensis represented in j)late LXI., fig. 5, of the 
“ F.A.S.,” compared with those of a very large fossil mandible of II. amp)hibius in 
the British Museum from Auvergne,® vis . : — 
H. sivalensis. 
H. amphibius. 
Length of symphysis 
7-1 
6-8 
Interval between canines .... 
80 
11-0 
Depth at pm. 1 
4-6 
5-1 
Longer diameter of canine .... 
2-4 
2-5 
,, ,, ,, 1. 1 . 
1-35 
1-4 
9 
1-0 
q 
1-3 
0-8 
Mandible of small variety . — In fig. 1 of plate VI. of this volume there is 
represented the left ramus of the mandible of a hippopotamus collected by Mr, 
Theobald in the Shvaliks of the Punjab, which differs somewhat from the typical 
mandibles of II. sivalensis^ as is shown by the following dimensions, vis . : — 
1 “F.A.S.,” pL LXL, fig. 4. 2 fig. 4a. 3 pi. LX., figs. 8, 9. 
4 Compare “F.A.S.,” pi. LXI., fig. 6, with BlainviUe’s “ Osteographie,” Gen. Hippopotamus, pi. II. 
5 B.M., Xo. 17,085. 
6 The homology of the lower incisors of II. amphibius will be discussed in the sequel. 
