PAL^.OMASTODON. 
133 
resulting from the gre<at development of the diploc. These cavities communicate witli 
the exterior by ])erforations of the outer table of the bone {for.). One of the largest 
of the openings is in the parietal, close to its junction with the stpiamosal and in the 
same vertical plane as the auditory meatus : the occurrence of similar openings in 
the skull of Mamthermm has been referred to. 
The squamosal [sq.) is a very large bone and forms the whole of the lateral 
})rominence at the posterior part of the skull. Posteriorly it unites with the exoccipital 
and above with the supraoccipital and parietal. In the specimen upon which this 
description is founded the upper portion is broken away, but it can be seen that it 
helped to form the lower part of the massive lambdoidal crest which runs down from 
the top of the skull to just above the auditory meatus, and that it is greatly thickened 
by the development of extensive air-sinuses. The posterior part of the bone which 
unites with the exoccipital behind forms a broad and deep post-tympanic flange {ply-), 
which, by meeting the posterior border of the glenoid surface, closes the external 
auditory meatus {e.a.m.) below. Externally the inferior edge of this post-tympanic 
process projects a little below the glenoid surface, forming a sort of false postglenoid 
process, but internally the border of the articular surface is more prominent and projects 
below it. Ill this inner portion it is possible that a narrow wedge of the tympanic bone 
may be interposed between the post-tympanic flange and the glenoid surface. This 
latter is very large and extends very high up in front ; it is gently concave from side 
to side and broadly convex from before backwards. This region differs considerably 
from that found in the later Elephants, in that the convexity of the glenoid surface 
continues up to the post-tympanic process, so that there is not, as in Elephas, a deeply 
concave area beneath the auditory meatus and behind the glenoid convexity (see text- 
fig. 49). The inner end of the articular surface forms a large prominence, to the inner 
side of which there is a deep fossa mainly excavated in the tympanic and referred to 
more fully in the account of that element. The outer side of the lateral prominence 
was prolonged forwards into a stout zygomatic process articulating with the jugal, 
which apparently extended beneath it nearly as far back as the hinder edge of the 
articular surface, of which it probably formed the outer part. In the later Elephants, 
owing to the great development of air-cells both in the squamosal itself and more 
especially in the surrounding bones, the zygomatic prominence is much less marked, 
and, in fact, the whole zygomatic arch is of much less importance. From the inner 
end of the glenoid surface the suture between the squamosal and the alisphenoid runs 
forwards for some distance, the squamosal having a somewhat greater extension on the 
side of the skull than in the recent Elephants. Anteriorly the squamosal joins the 
frontal for a short distance, and superiorly it meets the parietal in a nearly straight 
suture running downwards and forwards. 
The tympanic (text-flg. 49, ty.) is much less swollen than in Elephas and is a 
compressed mass of bone bounded posteriorly by the foramen lacerum poster ms {f.l.p.) 
