OCR Text |
Show 1899.] OSTEOLOGY OF THE TUBINARES. 399 The coracoid is of great width across the base. This is especially tbe case in those genera which have a notched sternum; in these, the shaft is shorter not only in proportion to the Avidth of the base, but also in proportion to the length of the sternum. The width of the base is relatively least in Pelecanoides, in which it does not exceed half the length of the shaft, and greatest in Diomedea, in which the breadth of the base and the length of the shaft are nearly equal. The procoracoid is large, aud there is a supracoracoid foramen. There is no articular facet on the acro-coracoid for the furculum. The scapula is subcylindrical and flattened at its free end, and about as long as the furculum measured from the hypo-cleideum across to the free end. The furculum is U-shaped, and Avith, or without, a hypocieideum. vii. THE PELVIC GIRDLE. The pelvic girdle of the Petrels most nearly resembles that of the Sphenisci. The resemblance in the case of the Diomedeidas, however, is less marked, as the pelvis, like the rest of the skeleton in this Family, is more specialized. In the Procellanidae, save in Ossifraga, the innominate bone remains free throughout life, and the pre-ilia do not meet in the mid-dorsal line above the synsacrum. The pre- and post-ilia are of about equal length. The ischium is produced far backwards and beyond the post-ilium, and turns sharply downwards to join the pubis, with which its free end is firmly united by ligament. The ilio-ischiadic foramen is large ; the obturator fissure is very wide and opens forward into the obturator foramen. The innominate of the Penguin differs from that of the Petrel in the smaller size of the ilio-ischiadic foramen, and the shorter and wider ischium. To the increase in the width of the latter the narrowness of the fissure is due. In Ossifraga the innominate is fused with the synsacrum, and the pre-ilia rise forwards to the level of the neural crest of the synsacrum. In Pelecanoides the pre-ilia are reduced to narrow bars of bone articulating Avith the extreme outer edge of the transverse synsacral ridge, whilst the pubis and ischium are produced directly backwards with a slight doAvnward curve precisely similar to that of the Alcidas, with which group they also agree in the great length of the posterior thoracic and sternal ribs, thus affordiuo* us another instance of the modification of parts by adaptation to similar functions. In the Diomedeidas the innominate is not only fused with the synsacrum, but the pre-ilia meet in the mid-dorsal line above its neural crest. The pelvis as a Avhole, on account of this, comes to resemble that of Sula. Other Ciconiiform resemblances have already been pointed out in describing the skull of this family. They suggest a parallel development of characters deriAed from a common source. |