SHUFELDT : OSTEOLOGY OF THE STEGANOPODES 125 



chiefly due to the broad sacrum, and not to the ilia, which bones, either in their 

 pre- or postacetabular parts, are quite narrow in their transverse direction. The 

 former area, in each one, being very slightly concaved, and in the latter, barely 

 convexed. Antitrochanterian processes are comparatively prominent, and each 

 overhangs a cotyloid cavity of moderate size, and of the usual ornithic character. 

 Laterally viewed, we note that the obturator foramen is complete, due to the fact 

 that either pubic style is in contact for nearly its entire length with the inferior 

 margin of the corresponding ischium. This obliterates anything like an obturator 

 space. Each pubic style is long and somewhat slender, and often passing an ischium 

 behind, curves downwards and very slightly inwards. Here the bone is appreciably 

 dilated in P. xthereus, which is not the case in the other species we are considering. 

 An ischium is also narrow, and projects some distance beyond the ilium of the same 

 side. A very shallow ilio-ischiac notch, on the posterior pelvic border, is formed 

 by this arrangement ; but by no means as well marked an one as we see in the 

 more typical steganopods, to be described later on in this memoir. 



The ilio-ischiac foramen is large and of a subelliptical outline (see Fig. 6, PI. XXL). 

 On the under side of the pelvis in Phaethon, we are to note the fact that the sacral ver- 

 tebrse, opposite the acetabula, do not throw out their processes to act as braces to 

 especially strengthen the ilio-pelvic walls at those points, a feature often seen in the 

 skeletons of other birds, and, as before found, both by Mivart and myself, to be the 

 case in Pelacanus, the true cormorants, in Sula, and to a somewhat less conspicuous 

 degree in Anhinga. 



Eight free caudal vertebrw, plus an elongated, ploughshare-shaped pygostyle, go to 

 make up the skeleton of the tail in a Tropic Bird. These vertebrse have spreading 

 diapophyses, especially the fifth one, and all have neural spines, while the last three 

 of them possess bifid haemal spines ; and this last character is impressed upon the 

 lower end of the coccyx. The neural canal passes through the entire series. From 

 this, and including the coccygeal piece, it will be seen, from our account of the spinal 

 column, as it has been given above, to contain in these birds, forty vertebrae. They are 

 all pneumatic, except the atlas and those which go to form the skeleton of the tail. 



The Sternum. — This bone in vay specimen of Phaethon fiavirostris has a body of a 

 quadrilateral form, being almost as wide as it is long. Its carina is deep anteriorly, 

 where it protrudes far forwards, while behind it merges with the ventral surface of 

 the body at some distance before it reaches the mid-xiphoidal process. In front the 

 carinal angle is thickened, and upon its superior aspect there exists a considerable 

 facet for articulation with infero-median surface of the os furcula. This facet is not 

 at the apex, but is found about one third the distance back towards the sternal body, 



