124 MEMOIES OF THE CARNEGIE MUSEUM 



SO. Finally, there are two pairs of thoracic ribs that are devoid of unciform proc- 

 esses, the costal ribs of the first of which reach the sternum, but those of the ulti- 

 mate pair fail to do so by some considerable distance in both species. From what 

 has been said in a foregoing paragraph, it will be seen that this gives one true pair 

 of " pelvic ribs," while the penultimate pleurapophyses and their hsemapophyses in 

 P. flavirostris articulate with a vertebra of the dorsum which is free, but which in P. 

 sethereus is coossified with the pelvic sacrum. So were we to have studied skeletons 

 of the latter species only, it would have been said that it possessed two pairs of 

 ''pelvic ribs," and had P. flavirostris alone been examined in this particular, the 

 descriptive osteologist would have recorded but one pair. This is but another 

 instance exemplifying the great necessity of having the proper material in sufficient 

 variety before one for comparison. 



Viewing the jjelvis from above, we find that the anterior margins of the ilia over- 

 lap the penultimate pair of thoraric ribs in the Red-billed Tropic Bird, not so, how- 

 ever, P. flavirostris ; and this, taken in connection with the apparent appropriation 

 on the part of the sacrum of one of the dorsal vertebrae, already referred to above, 

 in the case of the first-named species, lends to the pelvis a difference of appearance 

 in the skeletons of the two birds, now under consideration, which is not otherwise 

 particularly borne out by this bone. In part, the pelves of these specimens are other- 

 wise very much alike. Disregarding, then, the vertebra to which reference has just 

 been made, we find the sacrum to be composed of ten others, which are thoroughly 

 fused together and with the ilia, one upon either side. Taken as a whole, the pelvis 

 is broad, not especially long, and very shallow and compressed in the vertical direc- 

 tion. The internal borders of the ilia are nearly parallel to each other, and the 

 broad sacrum keeps them well apart for their entire length. Such an arrangement, 

 as we might expect, gives open, shallow " ilio-neural grooves," one on either side 

 of the sacral crista, anteriorly. These are quite continuous with similar " groov- 

 ings" carried back posteriorly as far as the first caudal vertebra. In other words, 

 between the inner borders of either ilium, and the supero-median line of the sacrum, 

 the diapophyses of the vertebrse are found to be below the general surface ; while 

 between them, for the entire length of the pelvis, occur foramina of some size, and 

 more or less subcircular in outline. These single parial rows of interdiapophysial 

 foramina constitute a striking character upon the superior aspect of the pelvis of 

 the Tropic Birds, inasmuch as in the preacetabular portion of that bone in Sula, 

 Plotis, the Pelicans, and the Cormorants, they are out of sight on this aspect by the 

 internal margins of the ilia meeting the sacral crista. Thus it will be appreciated 

 that the breadth of the pelvis in Pha'ethon, to which allusion has been made, is 



