Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/329

1871.] under surface of the skull is bounded by a raised broken edge directed downwards and backwards from the postorbital process towards the basisphenoid. The middle of this edge is crossed by a deep, narrow, horizontal groove continued forwards from a large foramen (v) situated nearly 1 inch behind. This groove may have lodged the ramus ophthalmicus of the 5th cranial nerve. All that part of the broken edge which is above and external to this groove is narrow ; it seems to be the remains of the thin bony plate separating the orbit from the temporal fossa ; while below and internal to the groove the edge widens into a broad, rough, four-sided mass having mesially and inferiorly the ascending basisphenoidal rod, in front the smooth under surface of the orbito-sphenoid, above the narrow groove for the ophthalmic branch of the 5th nerve, and behind a wide, shallow, vertical groove which descends from the large foramen lately mentioned. This latter, from its great size and its position, can be none other than the aperture which transmits the 5th nerve, the mandibular division of which was probably lodged in the wide shallow groove descending from it. That part of the side wall of the skull which lies in front of this foramen for the 5th nerve, behind the orbito-sphenoid, and which below joins the basisphenoid, must contain the alisphenoid. A narrow channel which ascends immediately behind the wide shallow groove for the mandibular nerve from near the posterior orifice of the carotid canal, and ends in a couple of small foramina nearly at the level of the floor of the cranial cavity, is probably arterial. One inch behind the foramen for the 5th nerve, and at the same level, but separated from it by a buttress, is a depression, at the bottom of which are two openings (fig. 1, vii). These I take to be the auditory fenestrae ; and if this view be right, the part of the side wall in front of them, and behind the foramen for the 5th nerve, contains the prootic bone ; while behind the auditory fenestrae a buttress ascending from the basis cranii towards the lower border of the suspensorium for the quadrate bone, in front of the exoccipital, has the relations of the opisthotic. In a triangular hollow between this and the exoccipital are the foramina for the 8th and 9th nerves (vii, ix).

The side walls of the skull behind the foramen for the fifth nerve, corresponding to the hinder half of the temporal fossa, slope outwards more than in front; they project beyond the auditory fenestrae, which they overhang, after the manner of the eaves of a house ; and beneath the overhanging eave, and nearly parallel with its outer border, is a wide shallow groove produced from the hollow containing the auditory fenestrae horizontally backwards and outwards to the root of the suspensorial process, which probably lodged the stapes.

The cranial cavity (fig. 2) is long and narrow. Its greatest transverse measurement is in front ; and its maximum vertical one is at the middle, nearly above the foramen for the fifth nerve. Two rather broad constrictions divide it into three fossae. Of these the anterior doubtless lodged the cerebral hemispheres ; it opens below into a remarkably large hypophysial pit (hpf). The posterior wall of this pit is vertical ; it makes a right angle with the floor of the cranial