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Show 398 MR. W. P. PYCRAFT ON T H E [Mar. 21, v. THE RIBS. The cervical ribs are styloid, and in the middle region of the neck are often of considerable length, e. g. Pelagodroma, Puffinus assimilis. They become very short posteriorly, and finally-on the 3 or 4 vertebrae preceding tbe thoracic-free. Anteriorly they fuse, above, with a process from beloAv the anterior zygapophysis (diapophysis), below with a ventral lamella running outwards and forwards from the centrum (parapophysis). Thus a canal is formed through Avhich the carotid passes. More or less Avell-marked catapophyses occur from the 6th to 10th vertebrae. There are 7 pairs of thoracic ribs, all of which, save the last, articulate by means of sternal segments with the sternum. The sternal ribs of the 7th pair are attached by ligament to the posterior border of those next in front. In Pelecanoides the thoracic and sternal ribs increase greatly in length from before backAvards so as to recall those of the Alcidas. In Pelecanoides and Diomedea only are two pairs of thoracic ribs overlapped by the ilium. Uncinates are present in all but the last one or two pairs. In Pelecanoides they are placed in the same horizontal plane and about halfway doAvn the shaft. In all other cases they are seated anteriorly low down, near the distal ^ of the rib and rise backAA*ards to about its middle. They are moderately long and slope obliquely upwards. In all cases they project beyond the rib next behind, and often extend to that succeeding this. A very useful table showing the number of the vertebrae, ribs, and uncinate processes in the different genera is given in Forbes's memoir (6). vi. THE STEBNUM AND PECTOEAL GIEDLE. The sternum assumes two forms-(1) that in which the posterior border is notched, and (2) that in which it is entire. The first includes all the genera except Pelecanoides, and the small forms included under sections A and B of the Procellariidas-e. £*., Procellaria, Oceanites. When the posterior border is notched, the anterior coracoid border is produced forwards far beyond the level of the anterior lateral processes. When the posterior border is entire, the anterior, coracoid border does not project far forwards. Pelecanoides belongs to this last division, but can at once be distinguished from the rest by reason of its great length in proportion to its width, in the feeble development of the spina externa, and in that the articular surfaces of the sternal ribs are confined to the free edge of the anterior lateral process. In Oceanites and Pelagodroma there is a large fenestra in the anterior dorsal region of the carina. As will be seen by the appended Key, the various genera which have a notched sternum can only very imperfectly be distinguished one from another. Pneumatic foramina opening on to the dorsal aspect of the sternum occur in Majaqueus, Priofinus, Ossifraga, and Diomedea. |