4S THE VOYAGE OF H.M.S. CHALLENGER. 
the posterior end of the sternum varies a great deal in its outline in different members 
of the group. 
In most of the larger forms of Procellariidee, the visceral aspect of the sternum pre- 
sents, towards its anterior extremity, more or fewer pneumatic apertures, which are best 
developed in the Albatrosses, where the whole bone is much permeated with air. In 
Fulmarus, and all the smaller forms of Procellariide, as well as in the Oceanitide, the 
sternum has no pneumatic apertures at all, and does not contain air. 
In the genera Cymochorea (fig. 5), Procellaria, and Halocyptena, as also in Fregetta 
(fig. 7) and Garrodia, the posterior margin of the sternum is entire, with only a very 
slight concavity in the outline of each side. In the Oceanitine genera Oceanites and 
Pelagodroma it is very nearly the same in shape, though each side has a small excava- 
tion, the margin between the excavations being convex. In Pelecanoides (fig. 3) also 
the sternum is nearly straight posteriorly. In the Diomedeine (e.g., Thalassiarche 
melanophrys, fig. 1) the posterior angles of the sternum are produced backwards and 
outwards considerably, and its posterior border is broadly excavated by a sinuous curve, 
convex externally, concave mesially. In Diomedea exulans the inner concave part is 
divided into two smaller concavities on each side by a process of bone, so that the 
posterior margin presents four slight notches, the inner pair being the bigger. 
In the remaining genera of the group, the posterior border of the sternum is always 
more or less four-notched posteriorly, the notches being generally best developed and 
deepest in the genera allied to Puffinus and Majaqueus (figs. 9-13), whilst in the 
Fulmarine forms the notches are smaller and tend to be irregular. Bulweria (fig. 16) 
departs widely from any of the so-called Storm-Petrels in the form of its sternum, 
and approaches closely (strelata, &e. The exact forms of this notching, which is 
inclined to vary in different specimens, may be best understood from an inspection of the 
figures. The outer notch may, as in the specimen of Adamastor figured (fig. 18), be 
converted into a foramen by the partial ossification of the membrane filling it. 
The coracoid bones in the smaller genera (vide Pl. VII. figs. 5-8) are well-developed, 
with nearly cylindrical shafts, dilated internally at their distal ends to meet the acromial 
process of the scapula, whilst proximally they are broad and expanded, and produced 
externally into a pointed, or slightly hooked process. In Pelecanoides (figs. 3, 4) these 
bones diverge at a smaller angle from each other than in the other forms; their shafts are 
less cylindrical, and the proximal ends comparatively little dilated. In the larger forms, 
on the other hand, the coracoids become very divergent, and the shaft and both 
extremities, but particularly the proximal one, are much dilated, so as to assume the 
extraordinary form seen in Diomedea (figs. 1 and 2) and its allies, where the greatest 
transverse diameter of the bone at its base is nearly as great as its entire length. The 
external outline of the bone is deeply concave, owing to the great development of its 
external costal process. 
