94 BULLETIN OF THE 



sulci of the frontal lobe are directed obliquely backwards to the longitudinal 

 fissure, thus reversing the direction observed in the recent ungulates. 



The Skull and Dentition. 



The skull (Plate III.) is broad in relation to its length, owing to the shorten- 

 ing of the ant-orbital region and the recession of the nasals. The maxillaries 

 spread very widely for the powerful series of molars, while the premaxillaries 

 are slender. The orbit is placed above the first molar. The nasals are com- 

 pressed anteriorly, and extend only so far as to overhang the premaxillary suture. 

 A marked feature of the skull is that the upper surface is in a nearly straight 

 line from the supra-occipital ridge to the tip of the nasals, while in A. megalodus 

 it is concave. The orbit is very slightly overhung by the supra-orbital process. 

 The zygomatic arch is deep vertically, but compressed laterally. The post- 

 glenoid process is deep and narrow ; it has contact with the post-tympanic of 

 variable length. The remarkable feature of the post-tympanic is its extension 

 into a broad flat plate behind the auditory meatus. The occiput is broad and 

 low, and does not overhang the condyles ; it is deeply cleft in the median line. 

 On the base of the skull, the foramina rotund um and spheno-orbitale are con- 

 fluent, as observed by Cope. The foramen ovale is either confluent with or 

 separated by a slender ridge of bone from the foramen lacerum medium. 



The molars and premolars are remarkable for the extreme flattening of the 

 outer surface of the ectoloph, all trace of the three vertical ridges having dis- 

 appeared. The first premolar is a simple 

 conical tooth implanted by a single fang ; it 

 is apparently inconstantly developed, for 

 Marsh makes no mention of it in his de- 

 scription of A. (ctcutum) fossiger. The inner 

 angles of the protoloph and metaloph unite 

 by the ' crochet ' in pm? and pm s to enclose 

 the median valley, as in Aceratherium. The 

 fourth premolar resembles the molars except 

 in the non-development of the 'anticrochet.' 

 The true molars are characterized as follows : 

 Figure 17. -First superior molar of b the constr j ct i on f tne inner portion of 

 AphdopsjassKjer X \. . , 



the protoloph into a separate column; by 



the strong development of the 'crochet,' which in m 1 and m 2 unites early with 

 the metaloph to enclose the anterior 'fossette'; by the development of the 

 * anticrochet ' at the inner angle of the metaloph and ectoloph; by the com- 

 plete enclosure of a posterior 'fossette' in the first and second molars. 



The inferior molars are of the simple rhinoceros pattern, there being no trace 

 of accessory folds. The first premolar is missing; the second is separated by a 

 rather narrow diastema from the large lateral tooth. Between the pair of large 

 semi-procumbent caniniform teeth are two small incisors. 



The lower jaws are very massive, with a strongly arched lower border. The 

 condyles are broad and elevated. The posterior border is broad, but not rugose. 





