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PROFESSOR FLOWER ON A NEWLY DISCOVERED 
III. That in which the outer wall is greatly developed, and in the main flat or smoothly 
convex, though with slight elevations and depressions, corresponding with those so 
regular and well marked in the last section. To this group Rhinoceros (including its 
various modifications) and Homalodontotherium belong. If a single upper molar of the 
last-mentioned form had only been discovered, it might almost have been referred to 
the genus Rhinoceros , using the term in a wide sense. It would, however, have been 
found to differ from the most typical forms of that type in the more smooth and regular 
convexity of the outer wall, the fuller development of the cingulum, the more complete 
union of the two inner columns, which intercepts the floor of the inlet to the median 
sinus, the less oblique direction of the posterior transverse ridge and consequent smaller 
size of the posterior sinus, and the absence of the ridge projecting into the median sinus 
called “ combing-plate.” 
The crowns of the lower molars of Perissodactyle Ungulates may, as is well known, 
be also derived from the same type as the upper teeth, i. e. two principal transverse 
ridges connecting two pairs of cusps ; but there are only two fundamental modifications. 
I. That in which this primitive form is retained, the ridges remaining transverse and 
unconnected with each other, as in the Lophiodons and Tapirs. II. That in which the 
ridges assume a crescentic form, their outer extremities curving forwards, so that the 
hinder ridge abuts against the external surface of the ridge in front of it. This is’ the 
case in all the remaining animals of the group. An unworn lower molar of a Rhino- 
ceros has thus externally two convex areas separated by a vertical- groove, and inter- 
nally two principal sinuses (see fig. 3, as and ms) corresponding to the projections exter- 
nally. The entrances to these sinuses are bordered by three conical pillars — the first 
(a t) of comparatively little importance, representing the anterior talon of the Tapir’s 
tooth, the second ( aic ) the largest, representing the antero-internal principal cusp, 
and the third (p i c) the postero-internal principal cusp. It is the large size and complex 
character of the last two, in addition to the excessive vertical lengthening of the crown, 
which distinguishes the Horse’s lower molar from that of the Paleeotherium and 
Rhinoceros. 
On comparing a lower molar of Homalodontotherium with an equally worn tooth of 
Rhinoceros , it will be seen that they are formed on precisely the same type. The only 
important differences are that the outer surface of the former is rather more flattened, 
the posterior convex area is relatively more elongated, being produced backwards into a 
sort of heel (p t) separated by a groove (p s ) on the inner side from the postero-internal 
column (p i c), and, as in the upper teeth, the cingulum is more completely developed. 
It differs from the regularly bicrescentic tooth of Paleeotherium still more than from 
that of Rhinoceros or Macrauchenia ; while in the lateral compression and flattening, and 
the complexity of the posterior column, it shows a slight approximation towards Equus. 
The molar and premolar teeth of both upper and lower jaws thus without question 
show strongly marked Rhinocerotic characters ; but on passing to the examination of 
the canines and incisors the resemblance completely fails, at least to the true Rhino- 
