112 
TERTIARY VERTEBRATA OF THE FAYtM. 
large round opening of the dental canal. Beneath this ridge the angular region is 
thin, concave on the inner face, and convex externally ; the angle itself is broadly 
rounded. On tlie outer face of the jaw, a little below and in front of the condyle, 
there is a ])roniinent rounded surface for the attachment of a powerful muscle. The 
ascending ramns, as a whole, is slightly inclined backwards. 
Lower Dentition (PI. XIV. figs. 1, 3, PI. XV. fig. 1 ; text-figs. 50, 53, 54). — The 
lower incisors (PI. XIV. fig. 3) are straight procumbent teeth, probably representing 
2 of the full series. Ventrally they are convex from side to side, dorsally concave ; 
the outer and inner borders are nearly straight and parallel in the unworn portion. 
The anterior end is worn above, so as to produce a chisel-shaped surface convex in 
outline ; the enamel covering the ventral face is comparatively thick, while on the dorsal 
face it is very thin and in some cases perhaps wanting. The outer edge of the tooth 
is comparatively thin, the inner much thicker. The two teeth are in close contact in 
the middle line and their concave upper surfaces together form a forward continuation 
of the spout-like symphysis. The ventral face is usually marked by a series of broad 
shallow longitudinal grooves. 
Only two premolars (text-figs. 53, 54) are present in the lower jaw. Of these 
3 is a comparatively small laterally compressed tooth consisting of a large 
pointed main cusp, which in some cases gives some indication of comprising two 
united elements : behind this there is a small obscure cusp and on the anterior and 
posterior ends of the tooth the cingulum forms slight projections. In the type 
specimen of P. headnelli this tooth is missing. Pni. 4 is much more complex, being 
a bilophodont tooth, each crest consisting of a pair of subequal cusps. There is also a 
small but distinct intermediate cirsp lying between and uniting the inner ends of the 
two outer main cusps. On the middle of the anterior end of the tooth is a closely 
adpressed cusp belonging to the cingulum, and towards tlie outer side of the posterior 
end a similar but much smaller element. The peculiarities of these premolars and 
their differences from the corresponding teeth of Moeritlterium seem to be the 
consequence of two distinct influences — one the tendency to the reduction and loss 
of the anterior members of the series, the other the tendency to the increasing 
complication of the posterior ones. The consequence of these influences is that^u?i. 2 
has disappeared altogether, 3 is reduced in relative size and sinq)ler than tlie 
corresponding tooth in Mceritherium, while pm. 4 is hilophodont and much more 
molariform than in the earlier genus. 
The molars (PI. XV. tig. 1 ; text-figs. 50, 53, 54) are all trilophodont, differing only 
in the several species in the degree to which the main cusps liccome subdivided 
and in the complication of the talon of m. 3. The anterior molar is much smaller 
than those behind. The following description refers especially to the type species, 
P. headnelii., the differences in the other species being noted below. 
The molars are trilophodont, each crest consisting of a pair of tubercles, of which 
