126 MEMOIRS OF THE CARNEGIE MUSEUM 



and the articulation, though very close and extensive, is free, and anchylosis has never 

 been seen by me to take place between the two bones. The coracoidal grooves decussate, 

 the median extremity of the right one passing beneath the median extremity of the left. 

 A very rudimentary manubrium is seen immediately below this point, and below it a 

 circular pneumatic foramen. Either costal process is broad and subtriangular in form, 

 and upon either costal border occur six facets for articulation with the hsemapoph- 

 yses which connect the sternum with the vertebral ribs. On the much-concaved 

 sternal body, just within the anterior border in the median line, dorsad, there runs 

 back longitudinally a characteristic bridge of bone, upon either side of which we 

 find a large subelliptical pneumatic foramen, and usually they are the only two 

 found in this situation. Other small ones occur, however, in the pitlets among the 

 hsemapophysial facets on the costal borders. Very broad external xiphoidal proc- 

 esses of the posterior part of the sternal body, extend further backwards than any 

 other part of the bone ; the middle xiphoidal process is broadly rounded. The in- 

 ternal pair of processes are small and delicate, and each one of the pair occupies a 

 mid-point in the great, round notch, on either side, existing between the mid- and 

 external xiphoidal process, thus creating a 4-notched sternum for this species, of a 

 peculiar pattern in this respect. Mivart has said in his "Axial Skeleton of the 

 Pelecanidfe" (T. Z. S., p. 365), that "in the Pelecanidx there is but a single lateral 

 xiphoid process on each side, while in Phaethon there are two on each side, in addi- 

 tion to the median xiphoid process." Farther on we shall see that this by no means 

 always holds good for P. tethereus. Passing next to the consideration of the sternum 

 in the species last mentioned, we observe that in the majority of its characters it 

 agrees with the bone as it has just been described for P. flavirostris. It is propor- 

 tionately larger however, and presents two striking differences. In the first place, 

 in all four of my specimens a large, blunt-pointed manubrium is present, which is 

 transversely pierced at its base b}^ a foramen. Again, the internal pair of xiphoidal 

 processes may be entirely absent, or only one may be present on one side, and when 

 such is the case, it appears only as a minute projection from the inner border of the 

 external xiphoidal process near its extremity. I have no specimens of the sternum 

 of P. sethereus wherein there is to be found a pair of internal xiphoidal processes, 

 and consequently its sternum is not four-notched as in its cogener, P. flavirostris. 

 More or less minute or scattered pneumatic foramina may be found on its thoracic 

 surface, above the two large ones which occupy similar positions to those noticed in 

 the Yellow-billed Tropic Bird. 



The os furcula is of the broad U-shaped pattern, with its median portion below, 

 widened, and on the under surface of which occurs the facet for articulation with the 



