OCR Text |
Show 384 MR. W. P. PYCRAFT ON T H E [Mar. 21, Diomedea exulans, in addition to the aperture just described, there is a second, opening immediately into the Eustachian groove. This takes the form of a deep cleft lying on either side of the rostrum. In other species of this genus and in Thalassogeron and Phcebetria the Eustachian groove is shallower and Avicler, and does not receive pneumatic apertures. The basitemporal plate of Pelecanoides differs markedly from that of all the rest of the group, in that it extends the whole width of the base of the skull lying between the quadrates. In all the other Petrels the angles of the triangular base are widely distant from the quadrate on either side. Pneumatic apertures such as those just described are wanting. Mammillary processes occur only in Ossifraga amongst the Procellariidas and in the Diomedeidas. In other members of the group the place which these occupy is indicated by a slight protuberance which is continued inwards to meet its felloAv of the opposite side in the form of a IOAV ridge. In Diomedea, Thalassogeron, and Ossifraga is a Avell-marked tubercle lying between the mammillary processes. This is absent in Phcebetria. A deep holloAV-the paroccipital notch-diAides the mammillary from the paroccipital processes, Avhich are moderately well developed, pneumatic, and with a sharp free edge. In the smaller Petrels, e. g. Oceanites, Pelagodroma, the outline of the basitemporal plate is continued directly backwards into these processes, there is no hollowing out at its base as in the larger species. A precondylar fossa is present in all, but is especially Avell-marked in the larger forms. The parasphenoidal rostrum is of uniform calibre throughout and terminates anteriorly in the form of a spine extending up to, or beyond, the leA*el of the mesethmoid. It may or may not support basipterygoid processes. These are largest in Ossifraga. In Fulmarus, Priocella, Daption, Pelecanoides, and CEstrelata they are still very distinct. In Puffinus they vary in size, from distinct processes to mere vestiges. In Procellaria and Cymodroma they are represented by minute prickles. In Pelagodroma and Oceanites and the Diomedeidas they are wanting entirely. The Lateral Aspect of the Cranium.-The tympanic cavity is in the dried skull represented by a small, shallow cavity bounded in front by the pneumatic aperture opeuing near the quadrate articular surface, above by the overhanging articular surface for the otic head of the quadrate, behind by the paroccipital process, and below by the mammillary process (when this is present). The fenestra ovale and the fenestra rotunda pierce the Avail of this cavity, opening immediately Avithin its mouth; behind and above these apertures is the mouth of a large pneumatic cavity leading upwards between the supra-occipital and the prootic bones. The temporalis recess is a large tubular cavity opening forwards above the articulation of the quadrate; it runs upwards under the temporal fossa to terminate near the middle line, in the region of the lambdoidal ridge. This recess is very small in Daption, |