258 OSTEOLOGY OF TUBINAEES AND STEGANOPODES. 



scries of true ribs occurring, connected to the sternum by costal ribs. 

 They support anchylosed epiplenral appendages, which may lap two 

 ribs in the middle of the series. 



A delicate pair of ribs also spring from beneath the ilia, but their 

 haemapophyses do not reach the costal borders of the sternum. 



These dorsal vertebrae are almost entirely devoid of metapophyses or 

 interlacing spiculae above. Their articulations are heteroccelous, and 

 they arc non-pneumatic throughout the series. 



In the pel r is the anterior ends of the ilia are truncate from before 

 backward, and their inner margins do not usually meet the neural crest 

 of the sacrum, leaving, in consequence, "ilio neural grooves" at their 

 sides. These bones are narrow, of nearly an equal width, and concave 

 in both directions. They rise over the acetabula, on either side, to 

 form small convex areas to the rear and above these cavities. Below 

 this the remainder of each ilium, aided by the corresponding ischium, 

 form the drooping sides of the pelvis so characteristic of the Petrels 



Each ischium has a posterior process, which, after forming the 

 superior boundary to the obturator space, curves downward to become 

 an expanded, foot-shaped end, which closes in the aforesaid space by 

 having the lower margin of the foot applied to the upper margin of the 

 post-pubis. 



The ischiatic foramen is nearly circular and, in common with the ace- 

 tabulum, rather small. 



There is no pro-pubis in this Fork-tailed Petrel, and the obturator 

 foramen and space, as a rule, form one vacuity. 



Eight spreading caudal vertebra', with a quadrilateral pygostyle, 

 make up the skeleton of the tail. There appear to be many more 

 of these segments, as the united urosacral vertebrae beyond them seem 

 to continue the series so perfectly. This is still more striking in Ful- 

 marus. 



Of the sternum and perioral arch. — The furcula is of the U' s ' ia P e ^ 

 variety, and curved backward toward the sternum, its quadrate hypoclei- 

 dium having its lower border usually in contact with the upper border 

 of the protruding carina! angle. 



The clavicular limbs are about of an equal width throughout, the 

 posterior tips of their heads becoming pointed, and in articulation 

 meet the scapular heads at their inner angles. The outer aspect of 

 each of these heads of the clavicular limbs are modeled so as to form 

 an extensive concave facet for the head of the corresponding coracoid. 

 This arrangement is not seen in the Albatrosses uor Fulmars. 



A coracoid is pi incipally notable for its greatly expanded sternal end, 

 the outer angle of which is much produced. 



Its shaft is subcylindrical and its head tuberous. The scapular proc- 

 ess descends upon the inner side of the shaft, but, so far as I can see, 

 is not pierced by any foramen, as it is in the Fulmars and Diomedea. 



A scapula is dilated posteriorly and truncate after the manner most 



