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Show 1S99.J FROM THE LONDON CLAY OF SHEPPEY. 783 no important information as to the affinities of the bird. It can, however, be seen that the bone is relatively rather short and stout, and that the outer surface of the trochanter is broad and flat and projects forward considerably in advance of the shaft. The proximal end of the tibia (/.) is also preserved, but is too imperfect for description. The cervical and dorsal vertebrae are represented by mere fragments, and the only other bone of the skeleton at all well preserved is the scapula (sc), tbe blade of which is nearly perfect. It lies in approximately its natural position nearly parallel to the vertebral column; its tip just overlaps the front of the pelvis, and its upper edge for a short distance conceals the lower border of the mandible. The portion preserved is slender, but less so than is the corresponding part of the scapula of Phaeihon; its distal end is slightly expanded. From this bone as here preserved no information of importance as to the affinities of the bird can be derived. Comparison of the pelvis of Prophaethon with those of other Steganopodes shows that in its general form it resembles that of Sula most nearly. The chief differences are that in the fossil the interosseous foramina are less distinct, the upper surface of the postacetabular region of the ilia more convex from side to side, and the pelvic escutcheon narrows less towards the hinder end. The pelves of Phalacrocorax and Plotus somewhat resemble the fossil in the expansion of the anterior end of the preacetabular ilia, but differ from it in the large size and number of the interosseous foramina which commence opposite the acetabulum, in the general form of the pelvic escutcheon, and in the presence of a sharp ridge (most prominent in Plotus) near the inner border of tbe postacetabular region of the ilium. From the pelves of Fregata and Phaeihon the fossil differs greatly. In both these genera the pelvis is very wide and shallow, and the ilia are widely separated throughout their length by the synsacral vertebrae, the transverse processes of which are exposed, or at least covered only with ossified fascia (e. g., in part of the preacetabular region of Phaeihon). In fact the fossil pelvis differs much more from those of Phaeihon and Fregata than from that of any other of the Steganopodes ; but since tbe skull shows conclusively that Prophaethon is by far most closely related to Phaeihon some explanation of this difference is necessary. If the pelves and hind limbs of Fregata and Phaeihon be examined, it will be found that, in proportion to the size of the body, they are very small and (dearly in a degenerate condition. The explanation of this seems to be that neither of these birds make use of their hind limbs nearly so much as the other Steganopodes, for although no member of the group employs its hind limbs to any great extent, all except Phaeihon and Fregata use them in swimming both on and under the surface of the water. I have lately had an excellent opportunity of observing tbe habits of both Frigate and Tropic birds, and I believe that they subsist entirely on surface-fish and |
