46 THE VOYAGE OF H.M.S. CHALLENGER. 



that of the cervical ones is always fifteen, as may be seen from the table appended 

 (p. 47). In the Oceanitidae, it will be observed, the number of cervico-dorsal vertebra; 

 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 vertebrae 

 has a w T ell developed free rib, which may have an uncinate process, and one or more of 

 the preceding vertebrae — usually two, but sometimes as many as four (Oceanites) — 

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

 (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 haA r e well-developed hypapophyses, 

 especially anteriorly. These are particularly strong and well-developed in Pelecanoides 

 as in other diving birds (e.g., Uria, 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 expression, of varying depth, at the bottom of which, in the largest species, 

 Dpi 'ii 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, aperture in 

 each centrum. The transverse processes, too, are in these latter birds very much hollowed 

 out for air cavities. 



The ribs in the Oceanitidae are peculiarly broad, and flattened out dorsally, to an 

 extent not seen in any Procellarian. 



In Pelecanoides the ribs are very long, and oblique in position, the more posterior 

 ones most so, with the angles formed by their vertebral and sternal moities very acute. 

 Thus the whole trunk almost becomes completely surrounded by a bony box, in a way 

 well calculated to resist the pressure of the water when these birds dive. The same 

 modification may be seen well-developed in the diving Alcidae (Uria, Alca, &c). 



The uncinate processes are well-developed and nearly straight. They are firmly 

 anchylosed to the ribs. 



As may be seen from the table, the number of ribs and uncinate processes varies 

 slightly, and the same is true for the sacral and caudal vertebrae. The latter have well- 



1 I count all those vertebra; which hear 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 rib-less vertebrae which are anchy- 

 losed together are " sacral,'' the remaining free ones " caudal." 



