142 TEETIAEY VEETEBEATA OF THE EATtTM. 



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 the outer face of the jaw, a little below and in front of the condyle, 

 there is a prominent rounded surface for the attachment of a powerful muscle. The 

 ascending ramus, as a whole, is slightly inclined backwards. 



Lower Dentition (PL 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 

 i. 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 

 pm. 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. Pm. 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 cusp 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 the 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 Moeritherium 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 jjm. 2 

 has disappeared altogether, pm. 3 is reduced in relative size and simpler than the 

 corresponding tooth in Moeritherium, while pm. 4 is bilophodont and much more 

 molariform than in the earlier genus. 



The molars (PL XV. fig. 1 ; text-figs. 50, 53, 54) are all trilophodont, difiiering only 

 in the, several species in the degree to which the main cusps become 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. headnelli, the differences in the other species being noted below. 



The molars are trilophodont, each crest consisting of a pair of tubercles, of which 



