142 MEMOIRS OF THE CARNEGIE MUSEUM 



continuous with each other. The lower one is a shelf-like projection with a convex 

 border forward and its articular surface in the horizontal plane. Immediately 

 above this rises a much broader surface, though not so long, which is decidedly con- 

 vex from above downward. This portion of the facet for the coracoid is consider- 

 ably higher than the plane in which the borders of the body of the bone are found. 

 It faces forward and outward, and has one regular convexity as its limiting margin 

 above. Between the point of its outer termination and the apex of the correspond- 

 ing costal process, the border is one sweeping concavity. 



This form of sternum is more or less peculiar to the Sulidx, and it departs in a 

 number of points from the form of the sternum of the Cormorants and of the Peli- 

 cans. Comparatively, the bone is not so long in Sula gossi as it is in S. bassana, S. 

 breivsieri and other species. Still the general pattern of the sternum is much the same 

 throughout the Sulidse. If we overlook the crossing of the coracoidal grooves in 

 the sternum of Phaethon xihereus, and its having a manubrium, there is a great 

 deal in the bone to remind us of the sternum of Snla gossi and, in fact, there are 

 more steganopodous characters in the sternum of the Tropic Bird than there are of 

 any other avian group with which I am at present acquainted. 



Of the Shoulder-girdle. (Plate XXIIL, Fig. 12.) This part of the skeleton is, 

 like so much of the rest of it, thoroughly pneumatic, the foramina occurring at 

 their usual sites. 



The clavicles form a broad U-shaped arch, being completely united below, Avhere, 

 at the median point beneath, they support an extensive facet for articulation with 

 the carinal angle of the sternum. This does away with any such thing as a hypo- 

 cleidium proper, notwithstanding the fact that the bone projects slightly over this facet. 



The clavicular limbs are compressed from side to side, broader above than below, 

 with their anterior and posterior borders rounded off. 



A clavicular head is also compressed in the same manner as its shaft, and 

 tapers off as a pointed process. 



The most striking feature of this part of the bone is, however, the extraordi- 

 nary facet it supports to articulate with the coracoid. Either one of these is situated 

 at its outer side, upon a promontory of bone which is found there, the latter being 

 of a proper form to receive it. The facet is of an elliptical outline, placed vertically, 

 and facing directly backward. Something of a notch is found between it and the 

 clavicular head, in which occurs a number of the principal pneumatic foramina of 

 the furcula. On the anterior surface, just below the summit of a coracoid, we find 

 a distinct elliptical facet for articulation with a similar one just described for the 

 fourchette. Between this and the ear-shaped glenoid facet a considerable valley 



