TilE VOYAGE OF THE ' CHALLENGER.' 417 



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 Ossifraya, 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 vertebrae varies from thirty-eight to 

 forty-two, but that of the cervical ones is always fifteen, as may be seen z o1 - Chall. 

 from the table appended (p. 419). In the Oceanitidae, it will be observed, pt^p.'le. 

 the number of cervico-dorsal vertebrae is twenty-one, in the Procellariidae 

 it is twenty-two, with two exceptions, where there are as many as 

 twenty- three. 



The articular cup of the atlas is always incomplete superiorly, the 

 odontoid process of the axis filling up the gap, and so completing the 

 joint. The fifteenth cervical vertebra has a well- developed free rib, 

 which may have an uncinate process, and one or more of the preceding 

 vertebras usually two, but sometimes as many as four (Oceanites) have 

 short V-shaped ribs, which do not anchylose with the vertebrae. Some- 

 times (Oceanites, Prion) the fourteenth cervical rib is longer, resembling 

 in shape that of the fifteenth, but with no uncinate process. 



The dorsal vertebrae * are all free, except the last, or occasionally two 

 last, which are anchylosed to those forming the sacrum. They usually 

 have well-developed hypapophyses, especially anteriorly. These are 

 particularly strong and well-developed in Pelecandides as in other diving 

 birds (e. g. Una, Alca, Podiceps), extending there to quite the last dorsal 

 vertebra. In the Diomedeinae, on the other hand, they are quite absent, 

 or merely represented, on the most anterior ones, by short expanded 

 processes like those of the few last cervical vertebrae. 



In nearly all the Tubinares, each of the dorsal vertebral centra has on 

 its sides a distinct oval depression, of varying depth, at the bottom of 

 which, in the largest species, open one or more small pneumatic foramina, 

 to admit air to the interior of the bones. In the Albatrosses, however, 

 these pneumatic depressions are absent, though air is admitted to the 

 bones which are highly pneumatic here by a distinct, but small, 



* I count all those vertebrae which bear ribs, whether true or false, behind the first 



dorsal defined as such by its rib being the first to articulate with the sternum as 



" dorsal." The succeeding ribless vertebrae which are anchylosed together are " sacral,' 

 the remaining free ones "caudal." 



2E 



