388 ME. AV. p. PXCRAFT ON THE [MoT. 21, 



in some Larid® and Alcidse, so that its presence is obviously of 

 no particular taxouomic value." Brandt (3) and Eheinhardt (17) 

 have made numerous and careful observations concerning this 

 bone. 



The Cranial Cavity. — The metencephalic j ossa 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 sellae. 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 ^Wth the floor of i\ie pituitary fossa and to its 

 outer side, lies the abducent foramen. 



The cerebellar fossa is bounded by the supraoccipital behind, the 

 pai-ietal above, and the prodtics below. In the Procellariidse 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 Diome- 

 deidae ; 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 procitics. 



The mtsenccplialic fossa lies in the alisphenoid and is moderately 

 deep; its superior external boundary is formed bj'^ the tentorial 

 ridge ; its ventri-lateral border is pierced in the Procellariida; by 

 the trigeminal foramen and the foramen for a branch of the vena 

 cejilialica 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 Diomedeidae 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 backwards. 

 The dorsum selhe overhangs it posteriorly, whilst the perpituitary 

 ridye bounds it in front ; this last is more or less flattened so as 

 to form an optic platform — representing the inferior border of 

 the ojytic foramen. Similarly, the upper boundary of the optic 

 platform is formed by a pre-optic ridge, which passes on either 

 side into the tentorial ridge. 



The optic foramen appears as a single aperture in manj'^ Procel- 

 lariidae ; in the rest, and in the Diomedeidae, it is more or less 

 completely divided into a right and left aperture by means of the 

 in t,er orbital septum. 



The cerebral fossa lie entirely in front of the cerebellar fossa, from 

 which they are separated by a ^^"ell-defined tentorial ridge. This may 

 be traced from the pre-optic platform outwards, backwards, 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 



