1888.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 267 



As a rule the anterior portion of eaeh ilium is depressed below the 

 neural crest of the united sacral vertebra, having "ilio-neural" canals to 

 their inner sides. Each anterior iliac border is truncate, thin, faintly 

 einarginated, and sometimes unevenly serrated, while the blade of the 

 bone is concave from before backward, as well as from side to side, and 

 contracts slightly before reaching the cotyloid cavity. 



Upon the under sides six vertebrae throw out their processes against 

 them, and both pairs of sacral ribs may become auchylosed, though 

 usually the anterior pair remain free, connecting with the sternum by 

 costal ribs. 



The seventh vertebra of the sacrum is the only one where the lateral 

 apophyses are reduced. lb is immediately opposite the acetabula. 



From the eighth, inclusive, and on, however, the plenr- and par- 

 apophyses regularly graduate into the form they assume in the true 

 caudals, their processes being very nearly of an equal length and their 

 extremities abutting against the free inner margins of the ilia. 



So really, from an under view, the sacral and caudal series of verte- 

 brae have all the appearance of a gradually modified chain of bones from 

 the last dorsal; in short, a tail with the pelvic bones simply pressed 

 against its sides to become anchylosed there. 



The post acetabular area of either ilium is very narrow and gradually 

 becomes reduced to a point behind, the surfaces turning toward each 

 other as we proceed in that direction, being bounded externally by a 

 raised border. This latter extends between the apex of each autitro- 

 chanter to the aforesaid point behind, where the ilia terminate posteri- 

 orly on either side. 



From this line and downward the sides of the pelvis are formed be- 

 hind by the remainder of each ilium, an ischium and a post-pubis. 



Fig. 10. Left lateral view of the pelvis and coccygeal vertebras of Fulmarus glaeiaUs rodgcrsii, d- Life 

 size. By the author. (Specimen 12G13, Smithsonian collection.) 



These surfaces look almost directly outward and only slightly upward. 

 Each presents for our inspection the large subelliptical ischiatic foramen, 

 the cotyloid cavity, and an ti trochanter; and the obturator foramen, 

 which here has almost entirely merged into the extensive and long, oval 

 obturator space, closed in behind, as in the Petrels, by the peculiarly 

 formed foot-like process of the hinder end of the ischium. 



