THE UPPER TRIASSIC OF WESTERN TEXAS. 79 



the sides slanting obliquely inward and downward until they meet in a thin edge 

 below; the extreme terminus of this edge is broken away. Posteriorly there is a break 

 between the edge and the anterior portion of the posterior part of the basisphenoid, 

 formed by the approximating edges of the basipterygoid processes. This break empha- 

 sizes the apparent bipartite character of the basisphenoid. On the sides of the basi- 

 sphenoid and marking the line between the two portions is a groove, which is very pro- 

 nounced below and becomes very shallow in its upper third. A little below the middle 

 of the course of the groove there is a deep extension forward of the cavity which runs a 

 little downward as well as forward. This cavity is so deep that exploration is difficult, 

 but apparently its anterior end communicates by a very small foramen with the hypo- 

 physial cavity below. Above this cavity the groove becomes very shallow and finally 

 terminates in a foramen which penetrates the wall of the brain-case. This foramen 

 is the common terminus of two leads, one the groove just described and the other a 

 foramen which in the specimen is first detected on the broken lower outer edge of the 

 opisthotic; the distal termination of the latter is not determinable. From its position 

 on the inner wall of the brain-case the common foramen is evidently the outlet for the 

 VII nerve. The deep vacuities at the middle of the course of the groove are probably 

 the inlets of the internal carotid arteries. Anterior to the grooves the sides of the 

 anterior portion of the basisphenoid contract rapidly into the thin ridge on the lower 

 edge; above this sharp edge and covered by it is a cavity, conical in form, with its posterior 

 end almost a point and the anterior part much larger in diameter. The anterior edge of 

 this opening is apparently coincident with an opening into the brain-cavity and so must 

 be the cavity for the pituitary body. On the sides of the bones, just about opposite 

 the cavity for the pituitary body, there are two small foramina on each side, probably 

 for blood-vessels. On the upper edge, just anterior to the openings of the VII nerves, 

 there is a large foramen on each side which can only be the opening for the V nerve. 

 The anterior portion of the basisphenoid described is in the position of the bone which 

 is called by Osborn the orbitosphenoid. 



On the inner side of the brain-case there is visible a decided angulation between 

 the parts of the floor formed by the basioccipital and the basisphenoid; near the middle 

 of the flat floor formed by the basisphenoid, but nearer the anterior than the posterior 

 edge, there is a pair of small foramina in the position usually occupied by the VI nerve. 

 The posterior part of the inner side of the brain-case wall is marked by a decided swelling 

 which sheltered the inner ear. The exact interpretation of this region is impossible, 

 because of the slight injury to the bone on both sides. Beneath the swelling marking 

 the position of the canals of the ear there is an opening of considerable size on both sides, 

 which leads directly into the large cavities on the sides of the skull at the inner end of 

 the opisthotic. This opening in all probability was the outlet for the IX-XI nerves. 



Just anterior to the swelling marking the position of the canals there is a shallow 

 notch with an elongate form, its greatest diameter vertical; it is apparent that the bottom 

 of this notch has not been reached in cleaning, and it seems very probable that there is 

 a small foramen leading from it into the region of the otic canals; if this is correct, this 

 notch marks the position of the escape of the VIII nerve; but a similar notch is shown by 

 Osborn, in his figure of Tyrannosaurus, just posterior to the otic region, which he regards 

 as marking an evagination of the dura mater into the cranial wall. Almost directly 

 above these notches and near the anterior edge of the preserved portion of the cranial 

 wall there is a second deep pit on each side which can not be traced through the bone 

 and appear to be blind; these are almost exactly in the position marked by Osborn, in 

 his figure of Tyrannosaurus, as other evaginations of the dura mater. 



This fragment of a skull is so radically different from that of the Phytosaurs found 

 in the same beds, and corresponds so well with the figures and descriptions, so far as they 

 have been given, of the same region in the primitive Theropodous Dinosaurs, that there 



