8 
INDIAN TERTIARY AND POST-TERTIARY VERTEBRATA. 
are invariably small and the outer large. The central pair are certainly incisors, 
and the outer pair also were formerly universally considered as such ; there are, 
however, strong reasons for considering that the latter may really be canines. Eor 
the sake of convenience, however, in this volume they will continue to be referred 
to as incisors, with this proviso. The lower molar series always consists of seven 
teeth, of which the first four are generally classed as premolars ; it is, however, 
quite possible that the first of these teeth, as in the upper jaw, is in reality a per- 
sistent milk-molar. Eour milk-molars are developed. 
The variations in the permanent dentition of Rhinoceros and Acerotherium 
may be expressed by the following general formula : — 
T (0—2) — (0—2) n 0—0 -ji/TTi/r (0—1) —(O—l) p-yr (3 —4) — (3 —4) -yr 3—3 
-*-* (0 — 2) — (0-2) ^'0-0 (0—1 ?) -(0—1?) J "(3?-4) — (3P-4) *3—3 
In describing the teeth of the various species in the sequel, the most anterior 
of the permanent molar series will usually be referred to as a premolar, irrespective 
of the question of its true homology. 
With regard to the structure of the upper molar teeth, the following terms are 
applied to the component parts in this memoir. The teeth are supposed to be 
viewed from the masticating and internal aspects, as in the accompanying figures : — ■ 
Anterior collis =. large antero-internal column. 
Posterior collis = large postero-internal column. 
Median valley = hollow dividing the two ‘ colies/ 
Anterior valley = hollow in front of ‘ anterior collis/ 
Posterior valley = hollow behind ‘ posterior collis/ 
Crochet = process projecting from the ‘ posterior collis 3 into the ■ median valley/ 
Combing-plate = process projecting from the outer wall of the tooth into the same. 
Ante-crochet — projection on the posterior side of the ‘ anterior collis/ 
Dorsum = outer surface of tooth. 
Costa = vertical ridge on c dorsum/ 
Pass = entrance into e median valley/ 
Buttress = projection at the antero-external angle of the crown. 
Accessory fossette = separate pit on the worn crown cut off from the outer extremity of 
the ‘ median valley/ 
Two different types of upper molars. — The upper molars of most species of 
Rhinoceros and Acerotherium easily fall into two main divisions, according to their 
structure. Those of one type, which may be called the Sumatran type, occur in the 
living Sumatran and Javan rhinoceroses, and are characterised by the production 
of the antero-external angle of the crown into a strong ‘ buttress,’ or column, which 
renders the outer wall of the tooth very sinuous. A characteristic tooth of this 
type is represented in figure 3 of plate III. No teeth of this type ever present 
an ‘accessory fossette’ caused by the union of the e crochet ’ and c combing-plate/ 
The second type of molar is represented in the large living Indian rhinoceros and 
in the African rhinoceroses ; it is characterised by the absence of the e buttress/ 
whence the external wall is approximately straight. Teeth of this type are repre- 
