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Show 1 020 MR. W. P. PYCRAFT ON THE [Dec. 19, In the Grebes, the squamoso-parietal wings do not attain to the height of the skull. In the larger species they cease abruptly at about halfway, a thin raised ridge running upwards from this point to the sagittal crest representing the lambdoidal ridge. In the smaller Grebes the squamoso-parietal wings are feebly developed. As in the larger species, the lambdoidal ridge is represented by a thin raised line, terminating at the sagittal crest. The Roof of the Cranium.-The parietal region in the Divers and larger Grebes is impressed by wide but shallow temporal fossae, divided, in the fully adult bird, by a narrow sagittal crest (Plate LXXII. tig. 1). It is interesting' to note that in a nearly full-grown skull of Colymbus septentrionalis in the Museum Collection this crest (Plate LXXII. fig. 2) is represented by a broad plate of bone, whilst iu an immature C. glacialis, apparently a little younger than that of C. sepjtentrionalis, inasmuch as some sutures are yet distinct, the sagittal crest is as sharply defined as in the adult. In the larger Grebes the sagittal crest is sharply defined, as are the temporal fossae. In the smaller species the anterior boundary-line of the temporal fossa is barely visible, posteriorly the fossa is moderately deep. The form of the fossa differs from that of the larger species and Divers, in that it is relatively shorter from before backwards, the cerebellar region of the skull only slightly projecting backwards beyond the cerebral. In the former the backward extension of the cerebellar prominence is very marked, more so than in the Penguins and Petrels. The supraorbital grooves of the frontals in the Divers are separated by a median knife-like edge ; externally, they are bounded by a broad supraorbital ledge the free edge of which is flattened, as in many Penguins. Anteriorly, the supraorbital ledge fuses with the posterior-dorsal limb of the lachrymal on either side. The anterior inner border of the ledge, immediately behind the lachrymal, is pierced by a large foramen for the passage of the lachrvmal duct. In the Grebes, the supraorbital region of the frontals is marked by a wide shallow median furrow; supraorbital grooves can hardly be said to exist, being represented only by a faint excavation along the free edge of the frontal. The supraorbital ledge, in the Divers, posteriorly combines with the alisphenoid, to form a prominent postorbital process. In the Grebes, this process is only very feebly developed; moreover, this region of the skull differs markedly in the two forms, in that, in the Divers the postorbital process is continued forwards as the supraorbital ledge, whilst in the Grebes the region in front of this process is marked by a shallow depression for the insertion of the muscles of the jaws. AZchmophorus seems to be the only Grebe to which the above remarks do not apply: iu this genus the periphery of the fossa lodging the muscle is produced outwards into a broad shelf-like emarginate postorbital process. |
