OCR Text |
Show 1022 MR. W. P. PYCRAFT ON THE [Dec. 19, The tympanic cavity in the Grebes is not so sharply defined as in the Divers, owing to the slighter development of the alisphenoidal wing of the parasphenoid; in other respects, however, it closely resembles that of the Divers. The squamosal prominence is only concerned with the suspension of the quadrate and does not, in addition-except very imperfectly, and only in the Divers-form the roof of a temporalis recess, as in the Tubinares and the Impennes. The paroccipital processes are well developed in the Colymbi, forming, as in the Impennes, a backward continuation of the squamosal prominence; in the Grebes (Podicipides) these processes are feebly developed, being represented ouly by an abruptly truncated lamina of bone. The temporal fossce.--In the Colymbi (Divers), and in the larger Podicipides (Grebes), the temporal fossae are very wide and shallow and sharply defined. They are separated in the mid-dorsal line-only by a sharp median sagittal crest. Within the confines of the" fossa, on each side, are defined the limits between the cerebral and cerebellar regions of the skull, the squamoso-parietal wdngs running transversely across the cerebellar dome, as in the Petrels, and not, as in the Penguins, traversing the boundary line between these two regions. In the smaller Grebes, e. g. Tachybaptes, the temporal fossa is only faintly defined, moreover it is almost entirely confined to the cerebral, and extends scarcely at all on to the cerebellar dome. This is due, not to any essential difference in the form or position of the fossa, but to the extremely slight development of the last-mentioned dome. The trigeminal foramen, in the Divers, lies immediately outside the inner wall of the mouth of the fossa representing the temporalis recess. There is no foramen for the sinus transversus branch of the vena cephalica posterior above this as in the Penguins and Petrels. In tbe Grebes the trigeminal and the venous foramen immediately above are confluent; it pierces the wall of the alisphenoid immediately in front of the articular surface for the head of the quadrate and below the anterior border of the squamosal prominence. It lies relatively further forwards than in the Divers, inasmuch as, in the Grebes, this foramen and the anterior border of the squamosal prominence lie only a short distance behind a vertical line passing through the postorbital process; in the Divers both these points lie far behind this line. The orbits, in the Divers, are overarched by the supraorbital ledges, the postorbital process bounding them posteriorly and the lachrymal anteriorly. The perforated interorbital septum forms a mesial partition-wall. In the Grebes the supraorbital ledges and postorbital processes are wanting, and the lachrymal is very small, so that the orbit in this group is not nearly so well defined. An osseous interorbital septum is wanting; in the Divers a large extent of the middle region is supplied by membrane, so that in the dried skull the septum is largely fenestrated. |