SHUPELDT : OSTEOLOGY OF THE STEGANOPODES 129 



Carpo-metacarpus presents us for examination the ornithic characters common to 

 many representives of this class of vertebrates. The coossified pollex metacarpal 

 is of average size, and the slender shaft of the medius metacarpal is, for the greater 

 part of its continuity, parallel with the far heavier shaft of index. This last, at the 

 anterior aspect of its distal end, develops a pronounced little process, which serves 

 to retain in their proper groove, the tendons which pass in that region during the 

 life of the individual. The free digit of pollex has the usual form seen in Aves 

 generally, and the expanded posterior portion of the proximal phalanx of the index 

 finger is entire, and not perforated as we find it in many of the Laridse. The distal 

 joint of this digit is long and slender, being once semibarbed near its lip, the pro- 

 jection standing out from its posterior angle. Long and spine-like, the free joint of 

 medius is seen to be proportionately more slender in the Red-billed than it is in the 

 Yellow-billed Tropic Bird. There do not appear to be any free terminal ungual 

 joints at the ends of any of the fingers in these birds ; and I have found no small 

 sesamoidal bones either about the carpus or near the elbow joint. 



The humerus alone of the bones of the pectoral limb is pneumatic. 



Passing to the consideration of the pelvic limh, we find that the femur too, is gener- 

 ally a pneumatic bone, and is peculiar inasmuch at the foramina occur irregularly at 

 the proximal end of the shaft either in front or behind, while in P. sethereus a single, 

 circular pneumatic foramen is found in the majority of skeletons in the popliteal fossa 

 near its center. For its proximal moiety, tibio-tarsus may likewise be hollow, and, 

 in life, air gains access to its interior. In one of my specimens a small perforating 

 foramen is seen between the low cnenial crests very near the summit of the bone. 

 Below the point indicated, the remainder of the skeleton of the pelvic limb seems 

 to be non-pneumatic. 



The axis of the head of the femur makes a wide obtuse angle with the longitu- 

 dinal axis of the shaft, and the pit for the insertion of the ligamentum teres is but 

 very faintly marked upon the top of it. On the summit of the bone we find the 

 usual articular surface, and above this the trochanterian crest is scarcely at all ele- 

 vated. At the proximal end of the shaft, between the great trochanter and the caput 

 femoris, a very appreciable fossa exists, and immediately below this a very distinct 

 tubercle presents itself in a great many specimens especially of Phaethon eethereus. It 

 at once reminds one of the trochanter miliar of the mammalia, and no such a char- 

 acter is ever to be seen upon the femora of any of the other steganopodous birds 

 judging from those before me, and I do not now recall ever having seen it upon the 

 femur of any other existing or extinct bird. This character is but faintly seen in 

 the U. S. National Museum specimen of the skeleton of P. flavirostris (No. 17841). 



