MCEEITHEEIUM. 101 



third molar, but in older skulls in whicli this tooth has come into wear the opening 

 is opposite its hinder end. The palate is narrow and the cheek-teeth are arranged in 

 straight and parallel series, 



The hasioGcipital (PL VIII. fig. 1 a, ho.) is broad and regularly convex from side to 

 side, at least posteriorly. Anteriorly the crushing that has been undergone obscures 

 both the form of the bone and its relations to the basisphenoid, nor are these points 

 better seen in other specimens. Posteriorly it is deeply notched by the ventral border 

 of the foramen magnum, and laterally it forms the lower ends of the occipital 

 condyles. External to the basioccipital is a large foramen, in part no doubt the 

 foramen lacerum posterius (f-l-P-) ; the postero-internal angle of this opening forms 

 a notch at the point of union of the basioccipital and exoccipital bones, probably 

 representing the condylar foramen. 



The exoccipitals (exo.) are very large, and the condyles are strongly convex from 

 above downwards, but less so from side to side. Above the condyles and over the 

 foramen magnum the bones rise nearly vertically, meeting in the middle line in a 

 suture about 3 cm. long, thus excluding the supraoccipital from the foramen. Laterally 

 and ventrally the exoccipital forms the inner and lower portion of a great flange of 

 bone, of which the outer part is formed by the squamosal. This plate of bone adds 

 greatly to the width and depth of the occipital surface of the skull, and its ventral 

 angle is the equivalent of the paroccipital process of the exoccipital (p.p.). In the 

 later type, Palceomastodon (PI. XII. fig. 1), the exoccipital region is drawn out in such 

 a way that the condyles project to a much greater degree behind the occipital surface, 

 which slopes forwards instead of being nearly vertical. At the same time the flange 

 of bone just described is, as it were, pulled out to form the surface lying between 

 the hinder border of the glenoid cavity and the occipital condyles, which surface is 

 likewise constituted by the squamosal and the paroccipital portion of the exoccipital 

 (see PI. XII. fig. 1, p.p.). In fact, the differences between this region of the skull in 

 Mceritherium and Palceomastodon are just such as would be produced, if the skull of 

 the former consisted of a plastic material and the condyles were pulled backwards so 

 as to lengthen the interval between them and the glenoid surface. As will be pointed 

 out in the description of the skull of Palceomastodon, the paroccipital process is there 

 represented by a blunt tuberosity only. 



The supraoccipital (soc.) is a large somewhat shield-shaped bone. Its upper 

 border, forming the upper portion of the lambdoidal crest, is nearly semicircular in 

 outline. At its summit the bone is greatly thickened (PI. VIII. fig. 1), and sends 

 forwards a triangular process between the hinder ends of the parietals, but there does 

 not appear to be any separate interparietal. The occipital surface is raised in the 

 middle line into a vertical ridge, on either side of which there is first a slight concavity 

 and then a convexity : this median ridge with its accompanying lateral depressions is 

 the predecessor of the great depression and ridge for muscle-attachment found in 



