BRAIX OF A FOSSIL UNGULATE. 883 



upper portion. It articulates with the parietals by a jagged 

 transverse suture anterior to this crest. The exoccipitals were 

 also large bones, including, as they no doubt did, the large con- 

 dyles and j^ractically the whole of the paroccipital processes. 

 They take no share in the lambdoid ridge, for their suture with 

 the mastoid and squamosal lies just posterior to this ridge. It is 

 a jagged line, starting at the base of the steep drop in the crest, 

 and, travelling behind it, runs down on to the paroccipital process. 

 The basioccipital is clearly marked oflF from the basisphenoid by a 

 slightly irregular groove. It is very wide behind, between the 

 paroccipital processes, and notched by the condylar foramina 

 which must pierce the skull between it and the exoccipitals (text- 

 fig. 153, p. 881). In front it is nai-rowed by the lacerate foramina 

 and by the gap in which the periotic is set. Its lower surface 

 is here heavily marked for muscular attachments. In section, 

 the bone is seen to be thin and dense posteriorly, but in front it 

 is thicker and more cancellous in structure (text-fig. 154). 



The pcii^ietals are remarkably extensive, and externally they 

 appear to form the entire cranial roof. Anteriorly, they cou- 

 siderably overlap the frontals, and the extent of this overlap can 

 be seen in section (text-iig. 154), where the junction of the bones 

 is indicated by a break in the structure of the roof (at " a " in the 

 figure). The parietals do not share in the formation of the lamb- 

 doid crest, but meet the supraoccipital anterior to it. There is no 

 trace of a distinct interparietal, and the strength of the sagittal 

 ridge has necessitated a firm union of the parietals. The suture 

 with the frontals is first seen as a jagged line on the postorbital 

 ridge. It travels forwards along this for about 12 mm., then 

 turns suddenly backward and curves down on the side of the 

 skull till it strikes the alisphenoid. Below, the parietal is 

 suturally connected with the alisphenoid along a short horizontal 

 line, and posteriorly it is considerably nari-owed by its bow-shaped 

 suture with the squamosal (text-fig. 150, p. 879). Inside the 

 skull, the fronto-parietal suture is seen as a very irregular line 

 running vertically on the wall of the cerebral fossa, and the 

 squamoso-parietal suture as an even more irregular line in front 

 of the tentorivim. Neither suture can be completely tracer!. 



The squamosal forms a considerable part of the side-wall of the 

 skull. Posteriorly it overgrows the mastoid, so that only a small 

 isolated surface of this bone is exposed. The suture with the 

 mastoid is anterior to the lambdoid crest, and as this bone gets 

 submerged the suture crosses the crest to form the junction with 

 the exoccipital. The squamosal sends a small vertical thin flange 

 on to the root of the paroccipital process. It is partly anchylosed 

 with the tympanic, and in front of the meatus is bored by the 

 very large post-glenoid foramen. Almost the whole length of the 

 bone is concerned in forming the zygomatic process, which is con- 

 cave from side to side above, has a vertical flange to the exterior, 

 and a flat glenoid sni-face below. The flange is I'oughened on its 

 upper edge for attachment of the temporal fascia. To the inside 



