420 ON THE PETRELS COLLECTED DUK1XG 



Zool. Chall. sternum is entire, with only a very slight concavity in the outline of each 

 pt^xi^p. 48 side. In the Oceanitine genera Oceanites and Pelayodroma it is very nearly 

 the same in shape, though each side has a small excavation, the margin 

 between the excavations being convex. In Pdecandides (PL XXIII. fig. 3) 

 also the sternum is nearly straight posteriorly. In the Diomedeiiiae (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 evulans the inner concave part is divided into 

 two smaller concavities on each side by a process of bone, so that the pos- 

 terior 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 (PI. XXIY. figs. 1-6), whilst in the Eulmarine 

 forms the notches are smaller and tend to be irregular. Bulweria (fig. 11) 

 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. 13), be converted into 

 a foramen by the partial ossification of the membrane filling it. 



The coracoid bones in the smaller genera (vide PL XXIV. figs. 7-10) 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 (PL XXIII. 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 trans- 

 verse 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. 



Ibid. p. 49. The scapula is a slender, slightly curved bone, presenting no special 

 peculiarities. Its acromial process is prolonged inwards and forwards 

 to nearly, or quite, meet the posterior end of the clavicle. The angle it 

 forms with the coracoid varies very much in different genera, being 

 most acute in Pelecanoides, whilst in the Oceanitidse it is hardly, if at all, 

 less than a right angle (vide PL XXIV. fig. 10, Fregetta). Procdlaria and 

 Cymochorea resemble the other Procellariida>, the coraco- scapular angle 

 being in them, obviously (t, c. fig. 8) acute. 



