36 INDIAN TERTIARY AND POST-TERTIARY YERTEBRATA. 
coincident with the long axis of the jaw. It is thus apparent that the differences 
existing between the teeth, which we have considered as the deciduous and per- 
manent molars of the fossil Siwalik rhinoceros, are paralleled by corresponding 
differences between the homologous teeth of a living species with very similarly formed 
teeth, and there is accordingly a presumption of the correctness of the reference. 
General remarks on the upper molar dentition of the Rhinocerotidce. — It has 
been shown from the preceding comparisons that in, at all events, some of the 
species of rhinoceros, whose upper molars are formed on the plan of those of the 
living Javan and Sumatran species, the milk-molars present a less degree of special- 
isation than the true molars in regard to the relative development of the so- 
called ‘ buttress ’ at the antero-external angle. If, however, we turn to other 
species, like R. indicus , in which there is no distinct ‘ buttress ’ developed in the 
upper true molars, it will be found that in the milk-molars there occurs an approach 
to this e buttress,’ very like that which occurs in the milk-molars of the R. suma- 
trensis type. 1 Indeed, in place of the very wide difference occurring between the 
true molars of R. indicus and R. sumatrensis, there is only a comparatively very 
slight difference between their milk-molars. Now, at all events, the greater number 
of the old forms of the Rhinocerotidse (Acer other ium, and I believe all the mio- 
cene species of Rhinoceros ) possess teeth of the Sumatran type, which approaches 
the type of the teeth of other old perissodactyles, such as Ralceotherium, Anchi- 
therium, Hyachyus, &c., and it is therefore pretty clear that this form of molar is 
the oldest. This, then, will explain the resemblance existing between the milk- 
molars of species whose true molars are formed, respectively, on the Sumatran and 
Indian types, it being not uncommon for ancestral characters to be retained in the 
deciduous series, which have long since disappeared in the permanent. It would 
be a necessary consequence of this hypothesis that species having teeth of the 
Indian type should be of comparatively recent origin, and such, indeed, appears to 
he the case. The earliest form seems to be the Siwalik R. platyrhinus, apparently 
only found in such parts of the Siwaliks as are without much doubt of pliocene 
age, and never occurring in the older Punjab and Sind beds; in the pleistocene we 
have R. tichorhinus, and at the present time R. indicus and R. simus. There also 
exist intermediate forms, which seem to have originated in the pliocene, and still live 
on; these have teeth without the distinct ‘buttress’ of the Sumatran type, but the 
‘ crochet ’ and ‘ combing-plate ’ do not unite to cut off an accessory ‘ fossette,’ as in the 
Indian type, and the latter may be absent ; these intermediate forms are exempli- 
fied by the fossil Rhinoceros etruscus, R. leptorhinus (Owen) and the living African 
R. hicornis. If these conclusions be true, the Sumatran and Javan rhinoceroses 
must be considered as being the little altered descendants of a very old type, 
while R. indicus is a much more specialised form of later origin. 
Mandibles of Siwalik rhinoceroses. — The authors of the “Eauna Antiqua 
1 This character is shown to a certain extent in the specimens of the milk-molar dentition of Ji. indicus figured 
by the author in the ‘Jour. As. Soc., Bengal’ (Vol. XLIX, pt. II, pi. VII), hut more clearly in still younger specimens. 
