REPORT ON THE ANATOMY OF THE PETRELS. 45 



though apparently not anchylosed with, the maxillary plates. The transition from this 

 to a desmognathons type would therefore be but very slight. 



The palatines in the smaller forms are of generally flat form, with their posterior 

 angles rounded off, closely apposed together for a very short way behind the posterior 

 nares, and with fairly developed descending and ascending plates, the latter being 

 recurved posteriorly and anchylosed to the vomer. This latter bone (PL VI. figs. 5 

 and 6) is always broad behind, of generally depressed form, and strongly bent down- 

 wards in front, its pointed extremity appearing between the maxillo-palatinc processes at 

 about their anterior edsje. 



In the larger forms the vomer becomes enormously broad, and keeled both above and 

 below. The palatines meet for a much more considerable distance posteriorly, greatest in 

 Fulmarus, and their descending plate becomes more pronounced ; at its anterior end the 

 bones of opposite sides nearly meet. The ascending plate, too, becomes very large, more 

 or less embracing the vomer at its base, and being separated, especially in the Albatrosses, 

 only by a narrow chink anteriorly from the posterior end of the equally upturned 

 maxillo-palatine. The posterior margin of the palatines is more or less concave. The 

 pterygoids are nearly straight, slightly compressed, cylindrical bones, which articulate 

 mesially partly with the basisphenoidal rostrum, partly with the truncated posterior 

 ends of the palatines. Well-developed basipterygoid facets are present in all the forms 

 (vide PI. VI. figs. 2 and 4), except the Diomedeina?, the Oceanitkke, Procellaria and 

 Cymochorea} The quadrate, as in most birds, is two-headed. Its distal end has two 

 distinct articular cartilage-coated areas, separated by a depression. The most external 

 of these is oblique from behind outwards, and is somewhat saddle-shaped, being convex 

 from side to side, and concave antero-posteriorly. The inner facet has its axis directed 

 forwards and inwards, nearly parallel to that of the pterygoid bone. It is divided by 

 a prominent oblique trochlea into an inner, nearly flat, surface, of triangular shape, 

 and a more external, deeply grooved one, also of saddle shape. As might be expected, 

 these features are less obvious in the feeble and smaller, than in the stronger and larger, 

 species of the group. 



The foramen magnum is more or less reniform, with the major axis transverse, in the 

 small species, whereas in the biggest it is oval, especially in Ossifraga, with the long axis 

 vertical. The moderately sized species are here again intermediate in structure. 



The mandible has no recurved angular process : its posterior end is more or less 

 inturned and truncated behind, the truncated surface being of triangular shape. The 

 articular surfaces are two in number, and, of course, of inverse shape to the corresponding 

 facets on the quadrate bone. One or more pneumatic foramina enter the bone at this 

 point. 



Axial skeleton. — The number of vertebra? varies from thirty-eight to forty-two, but 



1 Halocyptena has not yet been examined in this respect: it probably resembles the last two genera named. 



