48 tup: 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 Procellariidae, 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 Procellaxiidae, as well as in the Oceanitidaj, the 

 sternum has no pneumatic apertures at all, and does not contain air. 



In the genera Cymochorea (fig. 5), Proeellaria, and Ilcdoeyptena, 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 Oeeanites 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 Pelecano'ides (fig. 3) also 

 the sternum is nearly straight posteriorly. In the Diomedeinse (e.g., Thalassiarelie 

 melanoj)h)-i/s, 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 mesiaUy. In Diomedea exulahs 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 Pujjhnus 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 (Estrelata, &c. 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 PI. 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. 



