FAUNA OF THE G0SATJ FORMATION. 



635 



downward, and to have been largely removed by attrition, so that no 

 idea can now be formed of its relative depth. The presphenoidal part 

 of the specimen is broken away and terminates anteriorly in a trian- 

 gular transverse fracture (fig. 8). The base of the skull, as preserved, 

 is 5-| centim. long. In the middle it is 4 centim. wide, but narrows 

 posteriorly to the region of the condyle, where the bone is 33 millim. 

 wide. Anteriorly it also appears to contract a little ; but at both 

 ends the external white film of bone has scaled off, leaving the dark 

 phosphatic substance below — a condition reminding one curiously 

 of the pale and dark mineralization of bones in the Cambridge Green- 

 sand. This inferior region is concave in length, with a rounded 

 median ridge, and lateral concavities on each side of it in the middle. 

 Posterior to the middle area the bone is fractured interiorly for a 

 length of 2\ centim. ; and this fracture I suppose to have removed 

 the lower half and characteristic form of the occipital condyle. 



The basioccipital bone (PI. XXVII. fig. 7) terminates posteriorly in a 

 mass which, as preserved, is convex below and concave above, so as to 

 have a crescent outline, and is also moderately convex from side to side. 

 Above it is the brain-case, which certainly extended somewhat further 

 backward than the present limits of the occipital condyle, as is shown 

 by the form and character of the lateral walls. The superior surface, 

 however, of the occipital bone appears to curve convexly downward 

 as it extends backward ; and, as preserved, the bone is little over 

 1 centim. deep, and about 3 centim. wide at the origin of the con- 

 dyle, which is, as usual, defined by a lateral constriction that can 

 only be detected by careful examination, and is some distance pos- 

 terior to the lateral notches on each side of the base of the brain- 

 case. 



The cranial cavity (fig. 8) is imperfectly defined, because there is no 

 portion of the roof of the brain-case preserved, and its lateral walls 

 are imperfect. It is evident, however, that it is higher than wide ; 

 the posterior width in the region of the foramina for the hypoglossal 

 nerves is 28 millim., while the height appears to have been not less 

 than about 4 centim. On the right side a part of the inner wall of 

 the brain-case is exposed, showing that it is smooth, bulges inward 

 a little in the auditory region, and is inclined a little inward as it 

 extends upward. On the left side the bone is fractured, so as to 

 show that the hinder wall of the brain in the auditory region is 2 

 centim. thick, and extends outward transversely at a little higher 

 level than the base of the skull. It shows a horizontal semicircular 

 canal, which extends from the wall of the brain-case outward and 

 forward for a length of about 1^ centim., as exposed, and is 

 about | centim. wide. The curve cannot be followed round ; nor 

 can its relations to the other semicircular canals be definitely made 

 out. 



Along each side of the floor of the brain-case, and under the 

 transverse jutting of its lateral walls, which extend out horizontally 

 behind, is a row of foramina which extends in a curve, just sepa- 

 rated from each other by bony interspaces. Six are visible on the 

 left side ; on the right side there were certainly five, and may have 



