PETERSON: A. REVISION OF THE ENTELODONTIDA 79 
causes a characteristic trihedral cross-section, which is very much less apparent in 
the forms from the Oligocene. ‘The tooth is slightly larger than P+. ‘The entire 
crown is practically taken up by the large protocone. The cingulum has developed 
into @ narrow shelf on the antero-internal angle and continues in a much less con- 
spicuous manner on the internal face, terminating in the heavy cingulum posteriorly. 
P#* has no cingulum externally, which is also true of all the other premolars, and 
its transverse is slightly greater than its antero-posterior diameter. The tooth has 
a proto- and a deuterocone. The latter is relatively more developed than in Hntelo- 
don. There is a heavy cingulum posteriorly and on the antero-external angle a 
prominent basal heel is developed, but there are no external or internal cingula. 
The molars of Dinohyus are more specialized than those of Archxotheriwm, as 
isshown by the fact that the tubercles of the crowns are more depressed and consoli- 
dated, and the posterior portion of M* is relatively more developed than in the 
Oligocene genus. The crown of M+ in the type specimen is much worn, so that it 
shows two large and irregular tracts with the bottom of the cross-valley extending 
beyond the triturating surface, especially on the inner portion of the tooth. There 
are heavy cingula anteriorly and posteriorly and a less developed cingulum exter- 
nally, while internally the tooth is smooth. M+ is the largest of the series; the 
principal difference between it and the corresponding tooth in the Oligocene forms 
(especially those from the lower Oligocene) is revealed in the interruption of the 
internal exit of the cross-valley by a heavy rounded ridge (see Pl. LVIII, fig. 1) 
which was undoubtedly developed from the cingulum. The anterior border is 
entirely surrounded by a heavy cingulum, as is also the postero-external angle, while 
on the external face immediately back of the exit of the cross-valley there is very 
slight indication of a cingulum. ‘The posterior portion of M* is composed of (1) 
metacone, (2) hypocone, and (3) metaconule ; the first is of much larger size than the 
second, and the third is yery inconspicuous, in fact it is entirely absent on the corre- 
sponding molar of the right side of the type specimen. The cross-valley of M2 is 
interrupted in the same manner as is the case in the preceding tooth, and in addition 
this tooth has a mammillary cingulum which to some extent also closes up the 
external exit of the cross-valley. This cross-valley is not closed up in the speci- 
mens of Archxotheriwm known to me from the lower Oligocene. There is in the 
present type a heavy anterior cingulum on M2, but no posterior cingulum; the 
back part of the tooth having played a more important part in the make-up of the 
crown in this genus than is seen in older types. 
The antero-external tubercle of the last upper molar in Hntelodon magnum has 
reached a greater development than in Archxotherium and in this respect is more 
