SIWALIK AND NARBADA EQUlDiE. 
7—73 
horses consist mainly of teeth and jaws, it will avoid repetition to devote a few 
paragraphs to the consideration of the structure and number of the teeth in the two 
genera under consideration. In all the known genera of the family the whole 
complement of the full mammalian dentition is invariably developed, t]ie formula 
being — 
l-l 
4'4 
1 3-.3 Cl ri pm ' 4 4 ■ 3'3‘ 
The tooth here reckoned as the first premolar is in Equus and Hippotherimn, at all 
events, a milk-mo] ar, as it has neither predecessor nor successor ; it- is of small size, 
and, in most cases, disappears at an early age. It is analogous to the corresponding 
tooth in Rhinoceros. In Equus “ the molar teeth present an outer wall, which 
is bicrescentic in transverse section ; and two inner edges, which are curved more 
or less inwards and backwards, and correspond respectively with the anterior and 
posterior crescents of the outer wall. The valleys may be more or less completely 
filled up with cement, which also coats the tooth. The incisors are similar in form 
in each jaw, and in Equus and Rippotherium their crowns present a wide and deep 
median cavity, formed by a fold of enamel.” — (Huxley.) 
The structure of the molars may be more fully explained as follows, in the 
words of the same writer : — 
“ The outer wall of the tooth is bent in such a manner as to present from before backwards 
two concave surfaces, separated by a vertical ridge. From the anterior end, and from the middle 
of this outer wall, two laminae of the crown pass inwards and backwards, so as to be convex in- 
wards and concave outwards, and thus to include two spaces between themselves and the outer wall. 
From the inner surface of the hinder part of each of these crescentic laminae a vertical pillar is 
developed, and the inner surface of the pillar is grooved vertically. The outer wall, the laminae, 
and the pillars are all formed of dentine and enamel, thickly coated with cement. The attrition 
which takes place during mastication wears down the free surfaces of all these parts, so as, in the 
long run, to lay bare a surface of dentine in the middle of each, surrounded by a band of enamel, 
and outside this by the cement, with which the interspaces are filled. The band of enamel is 
simple and unplaited [in the young, hut somewhat plaited in the adult.] The general pattern of 
the worn crescentic surface may be described as consisting, externally of two longitudinal crescents, 
one behind the other, and with their concavities turned outwards. [In plate XIY, fig. 3 the 
component parts of the teeth are indicated in pm. 3. In this tooth a and b are the outer crescents, 
which will be termed below, respectively, the antei’ior and posterior, or first and second, outer 
crescents.] * * Internal to these, are two other crescents, partly transverse in direction, 
and connected by their anterior ends with the walls, which arise from the wear of the laminse ; 
[these crescents are indicated by the letters c and d, and will be termed, respectively, the anterior 
and posterior, or first and second inner crescents,] and attached to the inner surface of these two 
hour-glass-shaped surfaces produced by the wear of the two pillars. [These pillars are indicated, 
by the letters e and /, and will be designated, respectively, the anterior and posterior, or first and 
second pillars.] 
“ In the mandible the structure of the molars and the resulting pattern are quite different. 
[The right hand tooth in figure 5 of plate XII is selected to illustrate the structure of the lower 
molars.] The outer wall presents two convex surfaces, separated by a longitudinal depression, and 
thus reverses the conditions observable in the upper molars. The result of the wear of this is 
