OCR Text |
Show 388 MR, W. P. PYCRAFT ON THE [Mar. 21, in some Laridas and Alcidas, so that its presence is obviously of no particular taxonomic value." Brandt (3) and Bheinhardt (17) have made numerous and careful observations concerning this bone. The Cranial Cavity.-The metencephalic jossa takes the form of a moderately deep basin with gently sloping sides. It is steepest in front, where it rises to terminate at the dorsum sellas. In the posterior region, just behind and below the internal auditory meatus, lies the large vagus foramen, and further back, near the outer border of the occipital condyle, is the condyloid foramen. In the anterior region, near what one might call the " rim " of this basin, on a level with the floor of the pituitary fossa and to its outer side, lies the abducent foramen. The cerebellar fossa is bounded by the supraoccipital behind, the parietal above, and the prootics beloAv. In the Procellariidas the parietal portion is deeply corrugated, the ridges runningtransversely. These represent the sulci, and the corresponding depression the positions of the gyri. This feature is less marked in the Diomedeidas ; moreover, in the latter this fossa can be more or less distinctly divided into a median and two lateral regions, the latter lying above and in front of the prootics. The mesencephalic fossa lies in the alisphenoid and is moderately deep; its superior external boundary is formed by the tentorial ridge ; its ventri-lateral border is pierced in the Procellariidas by the trigeminal foramen and tbe foramen for a branch of the vena cephalica posterior. The former is the lower and opens externally just inside the ventral border of the mouth of the temporalis recess: the latter lies immediately above this and opens inside this recess. In the Diomedeidas these two foramina may have a common aperture which lies in a depression below that of, and leading into, the temporalis recess. The pituitary fossa is very deep and slopes obliquely backAvards. The dorsum sellce overhangs it posteriorly, whilst the perpituitary ridge bounds it in front; this last is more or less flattened so as to form an optic platform-representing the inferior border of the optic foramen. Similarly, the upper boundary of the optic platform is formed by a pre-optic ridge, AAiiich passes on either side into the tentorial ridge. The optic foramen appears as a single aperture in many Procellariidas ; in the rest, and in the Diomedeidas, it is more or less completely divided into a right and left aperture by means of the interorbital septum. The cerebral fossce lie entirely in front of the cerebellar fossa, from which they are separated by aAvell-defined tentorial ridge. This may be traced from the pre-optic platform outwards, backAvards, and upwards to a point iu the middle line, immediately above the centre of the floor of the metencephalic fossa, where it joins that of the other side. From the point of this junction there runs forwards, in the median line, a prominent ridge, the bony falx, which is continued forwards to the crista-galli, and marks the |