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 Oceanitidee, it will be observed, the number of cervico-dorsal vertebrae 
is twenty-one, in the Procellaridee, 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 vertebree 
has a well developed free rib, which may have an uncinate process, and one or more of 
the preceding vertebree—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 vertebree' 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 Pelecanoides 
as in other diving birds (e.g., Uria, Alca, Podiceps), extending there to quite the last 
dorsal vertebra. In the Diomedeine, 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 vertebree. 
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, 
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, aperture in 
each centrum. ‘The transverse processes, too, are in these latter birds very much hollowed 
out for air cavities. 
The ribs in the Oceanitidze 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 Alcidee (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- 
‘ 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 rib-less vertebrae which are anchy- 
losed together are “ sacral,” the remaining free ones “ caudal.” 
