36—2 INDIAN TERTIARY AND POST-TERTIARY VERTEBRATA. 
clieek-teeth of tlie pigs are remarkable for the circumstance that the first true molar 
becomes worn to a smooth surface of dentine before the last molar comes into use ; 
and this remarkable inequality in wear, together with the elongation of the latter 
tooth, which is very markedly developed in some forms, shows how easily a transition 
might have taken place from the teeth of a pig-like animal to those of a trilophodont 
mastodon : and from this, and other circumstances, the writer is strongly inclined to 
believe that the pigs and ])roboscidians were connected at no very distant (geological) 
date. 
A noteworthy circumstance is the disappearance at the present day of all the 
selenodont Suina ; this being accompanied by the disappearance of a Siwalik pig 
[Hippohyus), in which the dentition is of a more complex type than in any of its 
kindred. This disappearance may perhaps be explained by the probability 
that the more specialized selenodont Suina entered into competition with the 
still more sjDecialized ruminants, to which they had probably given origin, but were 
unable to stand against the advantages gained by the higher organization of that 
group ; while the less specialized group of Suina confined themselves more or less 
exclusively to swamps, and, therefore, did not enter into competition with the 
ruminants, and thus survived. 
/ 
Order : UNGULATA. Sub-Order : ARTIODACTYLA. 
Section; SUINA. Sub-Section: BUNODONTIA. 
Family I. : JIIPPOPOTAMIDjE. 
Genus: HIPPOPOTAMUS, Linn. 
Syn. Ch(8ropsis, Leidy ; llexaprotodon and Tetraprotodon^ Falc. 
Affinities . — The present genus, in which, as will be shown in the sequel, it 
appears desirable to include all the hippopotami, is apparently the only representative 
of the family, and is remarkably isolated, both in the present and past time. In 
respect of the form of the skull and mandible the genus comes nearest to 
Merycopotamus, and is thus intimately connected with the Anthracotlrn'idcP - : in the 
structure of its molar teeth it is, however, widely different, and in this respect seems 
to be nearest to the Siwalik genus Ilippohyus (described in the sequel), whose 
molar dentition, as remarked above, is of a peculiarly specialized structure. The 
molars of the hippopotamus also present a very remarkable resemblance to those of 
some of the Sirenia ; from which circumstance, together with certain resemblances 
in the structure of their brains, it has lately been suggested'^ that these animals are 
very closely allied. It is, however, just possible that this resemblance may be due to 
the somewhat similar mode of life of the hippopotamus and the Sirenia. 
1 It is in great part owing to this relationship that the writer divides the Artiodactyla into the Suina and the . 
Ruminantia, instead of into the Bunodontia and the Selenodontia. In the latter arrangement the hippopotamus and 
Merycopotamus would be widely separated. 
2 Chapman, ‘Pro. Acad. Philadel.,’ 1881, p. 126. 
