74—8 INDIAN TERTIARY AND POST-TERTIARY VERTEBRATA. 
necessarily, two crescents, the concavities of which are turned inwards, [These crescents are indi- 
cated hy the letters a and b in the figure, and will be alluded to respectively as the anterior and 
posterior, or first and second crescents.] A vertical pillar, longitudinally grooved on its inner face, 
is developed on the inner face of the tooth at the junction of the anterior and posterior crescent, 
and gives ris<f to a deeply bifurcated surface when worn. [This pillar is indicated hy the letter c. 
and will be termed the anterior or first pillar.] A second smaller pillar appears in connection with 
the inner face of the posterior end of the outer wall.'” [This pillar is indicated by the letter d, and 
will be termed the posterior or second pillar.] 
Homology qf component parts of molars . — The homology of the component 
parts of the equine molar with those of the molars of a less specialised type ( e.g ., 
Anchitherium and Rhinoceros) has been admirably pointed out hy M. Gaudry . 1 
That distinguished palaeontologist has shown that the outer c crescents ’ of the 
upper molars of the horse correspond to the two divisions of the outer wall of the 
rhinoceros molar , 2 while the inner | crescents 5 correspond to the areas connecting 
the two main ‘ colles ’ with the outer wall, and the two ‘ pillars ’ to the ‘ colles ’ 
themselves. The central spaces enclosed between the two transverse pairs of 
‘ crescents ’ in the equine molar, correspond to the median and posterior ‘ valleys ’ 
of the rhinoceros molar. In the lower molars the outer e crescents ’ of the equine 
tooth correspond to the greater part of the ‘ crescents ’ of the rhinoceros molar, while 
the c pillars ’ of the former correspond to the postero-internal extremities of the 
latter. 
Development. — The milk- dentition has the following formula : — 
. 3-3 n 44 
m.i.-g. 3 mm. 
The first true molar comes into use long before the milk-molars are shed. In the 
living horse the first milk-molar usually falls out at the time of appearance of the 
second premolar, and is never replaced. In some of the Indian domestic ponies 
this tooth is retained for a longer period, and we shall see such to have been the 
case among some of the Siwalik horses. The permanent canines are the last to 
appear, and they are not unfrequently never developed in the domesticated mare. 
“ The upper canines are distant from the outer incisors, while the lower canines are 
quite close to them. In both jaws there is a wide interval, or diastema , between 
the canines and the premolars.” 
Special characters. — The last molar is not more complex than the anterior teeth, 
nor the last milk-molar than the replacing premolar. This is a common perisso- 
dactyle, as distinguished from au artiodactyle, character. The molars and pre- 
molars do not form 6 fangs ’ till late in the life of the animal. 
In the horses and hippotheres the three last premolars are larger than the true 
molars, and the first premolar (milk-molar) very small, but in the older members 
of the family all the teeth of the molar series are more equal in size. 
Distinction between molars of TTtppotttktuttm and Equus. — The upper molars 
of Rippotlierium are distinguished from those of Equus, in an early state of wear, by 
1 “ Les Enchainements du Monde Animal, etc.,” Chap. V. 2 Vide preceding fasciculus. 
