48—14 INDIAN TERTIARY AND POST-TERTIARY VERTEBRATA. 
probable that tliese four species represent tlie actual line of descent in wliicli tliese 
modifications have been accoln 2 ^ 1 islled. 
Anotlier branch, which may have taken origin from the same hexaprotodont 
stock, is represented by 11. amphiUus., in which tlie symphysis is shorter than in 11. 
sivalensis, although longer than in the Narbada hippopotami : in this form the lower 
incisors are reduced to two, of which the inner is very large, and tlie outer very 
small. Finally in H. lihericnsis another tooth, probably corresponding to the small 
outer incisor of H. ampldbius^ has likewise disappeared. The longer symphysis of 
II. amphibius indicates that this species should not be regarded as the direct 
descendant of either of the Narbada hippopotami : the large size of iTa in II 
palceindicus still more strongly confirms this view in respect of that species. 
Homology of incisors. — In the case of 11. palceindicus it is jiroved that i72 is the 
tooth which is about to disappear ; and from the constancy of the order in which 
the terminal cheek-teeth tend to disappear among the more specialized mammals, it 
may be assumed with a high degree of probability that it is the same tootli which is 
wanting in the mandible of 11. amphibius. It is unfortunate that the upper incisive 
series of II. palceindicus is unknown, but from the disappearance of homologous 
cheek-teeth in the upper and lower jaws of mammals generally, it is probable that it 
is ^ which has disappeared in II. amphibius and the other tetraprotodont forms. 
This inference is confirmed by certain observations recorded by Prof. P. Albrecht,^ 
which lead to the conclusion that it is the same tooth which has disappeared in man, 
and, therefore, in other mammals.^ 
The small size of iTs in II. amphibius renders it probable that it is this tooth 
which has disappeared in U. liberiensis ; and it would, therefore, seem that in those 
mammals where the incisors are reduced to a single pair, that pair corresponds to the 
first of the typical series. From the instance of II. liberiensis it also seems that the 
disappearance of each pair of incisors commences in the lower, sooner than in the 
upper jaw ; this arrangement corresponding with that which obtains in the case of 
the cheek-teeth among mammals generally. 
If these conclusions hold good the two upper incisors which are occasionally 
developed in Rhinoceros^ will be respectively the first and third of the eutherian 
series ; the single tooth that is normally present being the first. It will also be 
evident that the two pairs of lower cutting teeth which are frequently present in 
that genus, will be respectively the inner incisor and the canine ; since if they were 
both incisors, as was formerly considered to be the case, the number of lower incisors 
would exceed the upper ; which is aj^parently never the case among the eutherian 
1 “ Sur les 4 Os Intermaxillaires, le Bec-de-Lievre, et la Valeur Morphologique des Dents Superieures de rHomnic,” 
Brussels, 1883. 
2 The instances of Ursus labiatus in which m is wanting, and Babirussa in which i:_3 has disappeared, are exceptions ; 
hut as the space for this tooth persists in the former, and the dentition of the species is altogether abnormal, while in the 
latter the disappearance of ^ is clearly due to the large canine, these exceptions need not invalidate the rule. 
3 Vide ‘ Journ. As. Soc. Beng.,” vol. XLIX., pt. II., p. 135, pi. VII., fig. 1 : these teeth are numbered l and i- 2 in 
