SKULL OF DINOTHfiRlUM GTGANTEUM. 531 



The premaxillse (pmx.), though narrower than the maxilJas, are 

 still very massive and widely expanded. Their upper surface is 

 concave from side to side, the lower convex in the same direction. 

 In the mid-ventral line they are separated in the actual skull by 

 a cleft, probably the result of distortion through crushing : in the 

 cast, which seems to have been made before the skull had been 

 broken, there is only a deep groove. From the posterior angles of 

 this groove a pair of ridges run back on the palatal surface to the 

 anterior end of the tooth series. 



The anterior ends of the premaxillse are thickened, and terminate 

 in a nearly flat surface in which there aie some irregular asym- 

 metrically arranged pits but no real trace of any incisor alveoli, 

 though it seems not unlikely that incisors will be found to have 

 been present in the young animal. The anterior portion of the 

 skull in front of the tooth series is curved downwards, its palatal 

 surface approximately following the curve of the downturned 

 mandibular symphysis. 



The structure of the basal region of the skull is not very clear, 

 probably in consequence of the crushing and fracturing it has 

 undergone. On either side of the basioccipital there is a fairly 

 well- developed auditory bulla (t.b.). External to this is a strong 

 crest of bone, terminating posteriorly at the process described 

 above as the paroccipital (p.p.). Anteriorly it runs inwards to 

 the sides of the basisphenoid. This crest, which seems — at least in 

 front — to be formed by the pterygoid, turns downwai'ds anterior])', 

 and forms the border of the narrow opening of the posterior 

 nares (p.n.) : ventrally it terminates jxist behind the tooth series 

 in a boss of bone which forms the posterior angle of the hard 

 palate. Dorsal to this boss there is a deep fossa, presumably 

 in the palatine, passing upwards to the hinder opening of the ali- 

 sphenoid canal (al.c). Behind and a little to the outer side of this 

 opening is the inner end of the extraordinarily elongated glenoid 

 surface (gl.) for the mandible. This surface is very narrow 

 from before backwards and gently convex in the same direction : 

 posteriorly it is bounded by the depression separating it from the 

 post-tympanic flange. Its outer end is immediately behind the 

 base of the zygomatic process (zi/g.) of the squamosal, and its 

 anterior border, at least in its outer half, abuts on the temporal 

 fossa. The auditory opening («m.o.) is opposite the outer border 

 of the zygomatic process : the external auditory meatus is greatly 

 elongated, and its floor is formed by the roof of the channel 

 behind the glenoid surface. Above the auditory opening the 

 massive lateral angle of the squamosal (a.s.) projects outwards some 

 fourteen or fifteen centimetres. The structure of a bony laby- 

 rinth supposed to belong to Dinotherium has been described and 

 figured by Claudius (" Das Gehorlabyrinth von Dmother'mm 

 giganteum" Palseontographica, vol. xiii. (1864-66) p. 65). 



The opening of the optic foramen is indicated by a depression, 

 from the upper edge of which a downwardly deflected ridge {or.r.) 

 runs upwards and forwards across the side wall of the skull and 



