AESINOITHEEIUM. lid 



The mandible differs from that of the Dinocerata in being more massively built, 

 in the absence of downwardly directed flanges of bone, in the absence of a diastema, 

 and in the much greater depth and different form of the posterior part of the jaw. 



Lower Peimanent Dentition. — Although in no specimen are the teeth perfectly 

 preserved in the front of the mandible (PI. IV. figs. 1, 3 ; and text-fig. 6, B), it can be 

 determined with certainty that the dentition was complete (^. 3, c. 1, pm. 4, 7n. 3). 

 The teeth all have high crowns, wear to a common level, and form a closed series, the 

 median incisors being in close contact with one another in the middle line. 



The molars, like those of the upper jaw, are extremely hypsodont, especially on 

 their outer side, where the enamel-covered portion of the crown is much higher than 

 on the inner side. The unworn molars are bilophodont (PL V. fig. 4), consisting of 

 two high narrow transverse crests, which are connected by a much lower oblique 

 crest running from the summit of the outer end of the posterior column to the 

 base of the inner end of the anterior column. A similar ridge also runs from the 

 outer end of the anterior column forwards and inwards, forming the anterior border 

 of the tooth, and enclosing with the anterior face of the front column a shallow 

 depression. The tooth may, in fact, be regarded as composed of two V-shaped 

 columns, the posterior limbs of the V's being placed transversely and being much 

 thicker and higher than the comparatively slightly developed anterior limbs. In 

 wear this structure becomes more obvious. The anterior face of the transverse crests 

 (that is, the sides of the V's facing inwards) is covered by very thin enamel, differing 

 entirely from that coating the outer side of the tooth. In the last molar the talon 

 is represented by one or two irregular, small, but prominent cusps. The cingulum is 

 well developed on the posterior face of the molars, where it forms a distinct shelf-like 

 projection. 



The phases of wear in the molars are, seen in PI. IV. figs. 1, 3. In m. 3 

 the two main columns are shown with the anterior arm of the front V just 

 in wear and enclosing a shallow fossa in front of the tooth. In the next the 

 anterior arm of the posterior V is strongly worn, and the anterior fossa just 

 referred to has been completely lost on the right side, but is still represented on 

 the left by a small island of enamel. In both these teeth the inner face is still 

 covered with enamel, though the covering is much thinner than on the outer side. 

 In m. 1 the inner side of the tooth has already been worn down below the level 

 of the crown, so that the enamel is there wanting. The divisions between the 

 two arms of the V's have now quite disappeared, but the outer ends of the two 

 main columns are still separated by a fold of some depth, which also disappears 

 when the tooth is a little more worn. The inner edge of the teeth is considerably 

 higher than the outer, in correlation with the opposite condition in the opposing 

 upper molars. 



The premolars differ greatly from the molars, their crowns being much narrower 



