| OCR Text |
Show 778 MR. C W. ANDREWS ON A NEW BIRD [June 20, hemispherical, its upper border being very slightly flattened. The foramen magnum is relatively large and is subcircular, the lateral and ventral borders being slightly flattened. Above the foramen there is a well-marked cerebellar prominence (cb.p.) from which the bone has been almost entirely broken away. On either side of the prominence, about 4 m m . above the foramen magnum, is a small vascular foramen from which a groove runs downward and outwards to the base of the paroccipital process (p.p.). These processes are large and convex from above downwards; tbeir rounded outer extremities do not extend below the level of the foramen magnum and their ventral border is continuous with the supra - foraminal ridge. Beneath the paroccipital processes the occipital surface is flattened and is produced downward beneath tbe occipital condyle in a pair of prominences, the extremities of which form the mammillary tuberosities (m.t.). About on the level of the occipital condyle there are several foramina (transmitting the hypoglossal, pneumogastric, and glossopharyngeal nerves, and ? carotid artery). The lambdoidal ridge, which forms the dorsal border of the occipital surface, is much worn away, but probably was never very strongly marked ; near its lower end it joins the ridge forming the outer border of the paroccipital process and then runs forward on to the zvgomatic process. This projects strongly forward towards tbe postorbital process (p.o.p.), from which it is separated by a space of about 6 m m . only. The temporal fossa (*./.) is very deep and well-defined; it extends scarcely at all on to the roof of the skull. The temporal ridge joins the lambdoidal crest near its outer end and then passes forward and inward : anteriorly it passes on to the postorbital process, along the middle of which it runs and at the tip of which it terminates. The parietal region of the skull between the temporal fossae is only very slightly convex, but in the frontal region between the postorbital processes the convexity is greater, and there is a pair of slight prominences separated in the middle line by a shallow depression which broadens out till in the interorbital region the whole roof of the skull is slightly concave from side to side. The orbital borders of the frontals are thin, sharp, and slightly upturned, but in front of the orbits the edges of the skull-roof become thickened and form a broad surface for union with the lachrymals (l.s.), which unfortunately are both wanting in this specimen. The region between tbe lachrymals is somewmat swollen, slightly convex from side to side and strongly so from before backward ; this inflated region terminates anteriorly in a deep transverse groove, the so-called naso-frontal hinge (r.h.). As a matter of fact this groove does not occur at the junction of the nasals and frontals, at least in Phaethon, to which the present species is most nearly allied, in the skull of a young individual of P. cethereus described by Mr. W . P. Pycraft, the groove divides the nasals into a posterior inflated portion and an anterior region cleft by the nares and separated one from another by the facial processes of the pre-maxillae; it will therefore be better to speak of this hinge as " rostral " instead of fronto-nasal. |
