30 PROCEEDINGS OF THE GEOLOGICAL society. [June 16, 
excavated the upper jaw, above it, but found no trace of a successional 
tooth ; the air-cavities of the maxillary sinus were exposed. 
The animal to which this cranium belonged was not quite of mature 
age when it perished. The last true molar M3, though through 
the gum, has had the summits of the outer crescents very slightly 
abraded ; and the first (P 2) and second (P 3) premolars in place are 
but little worn. The characters of the grinding surfaces are, there- 
fore, beautifully manifested, and in degrees of attrition that almost 
correspond with those of the upper molars of the Paleotherium 
crassum figured by Cuvier in the ‘Ossemens Fossiles,’ t. ii. pl. 48. 
fig. 2; with which, therefore, I shall proceed to compare the molars 
of the Paloplotherium. The antero-posterior extent of the six 
molars of Paleotherium crassum (the first bemg wanting) is 4 inches 
55 lines (11 centimeters 3 millimeters, Fr.), that of the six molars 
(the entire series) in Paloplotherium (PI. III. figs. 1 & 2, M 3 to P-2) 
is 3 inches | line (7 centimeters 7 millimeters, Fr.). The first grinder 
in Paloplotherium answers to the second premolar in Paleotherium : 
its antero-posterior extent is 9 millimeters, that of the second pre- 
molar in Pal. crassum is 16 millimeters. 
This tooth P 2, in Paloplotherium (ib. figs. 1 & 2), presents, like 
P 1 in Paleotherium, one conical lobe externally: there is a second 
smaller compressed lobe p at the fore-part of the inner side of the 
tooth, and the crown is surrounded by a strong basal ridge, inter- 
rupted only at the anterior angle of the inner lobe and at the back 
part of the tooth in contact with the next. The form of the grmding 
surface is triangular, broad, and excavated behind ; it is implanted by 
three fangs: but in its general character P 2 in Paloplotherium cor- 
responds with P 1 in Paleotherium magnum* ; with this difference, 
that there is an inner lobe near the back part of the grinding surface 
in Pal. magnum. 
The second premolar in the upper jaw of Paloplotherium differs 
from both P 2m Paleotherium+ and its proper homologue P 3 in 
Paleotherium, in having, like the tooth before it, but one external 
conical lobe ; it presents, however, a third lobule at the fore-part of 
the crown, wedged between the outer lobe o and inner tubercle p: 
this tubercle is relatively thicker than in p 2, but is still at the fore- 
part of the inner half of the grinding surface: the back part being 
widely excavated ; but showing a rudimental oblique tubercle (@). 
The basal ridge is mterrupted at the inner side of the base of the 
inner lobe. This tooth is implanted by three fangs. 
The last upper premolar (P 4) in Paloplotherium resembles each 
of the three posterior premolars of Paleotherium in having, like the 
true molars, two exterior conical crescentic lobes 0, 0’, divided by a 
vertical ridge extending from the external basal ridge, but not quite 
reaching the summit of the grinding surface ; a second parallel ridge 
bounds the fore-part of the outer surface of this premolar, as in Paleo- 
* Cuyv. J. ec. pl. 43. fig. 1, 3b. 
+ See my ‘ Odontography,’ pl. 135. figs. 5 & 6, where the homologies of the 
individual molars are determined, and expressed by the letters and numbers used 
throughout the present memoir. 
