384 MB. W. p. PTCEA'PT ON THE [Mar. 21, 



Diomedea exulans, in addition to the aperture just described, there 

 is a second, o])ening 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 ThaJassogeron and 

 Phcehetria the Eustachian groove is shallower and wider, and does 

 not receive pneumatic apertures. 



The hasiUmporal jilate of Peleccmokle.s diifers 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 ai'e wanting. 



Mammillary processes occur only in Ossifraga amongst the 

 Procellariidse and in the Diomedeidte. In other members of the 

 group the place which these occupy is indicated by a slight pro- 

 tuberance which is continued inwards to meet its fellow of the 

 opposite side in the form of a low ridge. In JDiomedea, TJialasso- 

 geron, and Ossifraga is a well-marked tubercle lying between the 

 mammillary processes. This is absent in Phcehetria. A deep 

 hollow — the paroccipital notch — divides the mammillary from the 

 paroccipital processes, which are moderately well developed, 

 pneumatic, and A\ith a sharp free edge. In the smaller Petrels, 

 e. g. Oceanifes, 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 precondylnr 

 fossa is present in all, but is especially well-marked in the larger 

 forms. 



The parasphenoidal rostrum is of uniform calibre throughout 

 and terminates anteriorlv in the form of a spine extending up to, 

 or beyond, the level of the mesethmoid. It may or may not 

 support basipterygoid processes. These are largest in Ossifraga. 

 In Fuhnarus, Priocella, Daption, Pelecanoides, and (Estrelata 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 Diomedeidte they are wanting entirely. 



The Lateral Aspect of the Oranium. — The tympanic cavity is in 

 the dried skull represented by a small, shallow cavity bounded 

 in front by the pneumatic aperture opening near the quadrate 

 articular surface, above by the overhanging articidar 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 waU of this cavity, 

 opening immediately within 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, 



