OCR Text |
Show 1903.] OSTEOLOGY OF THE CUCULIFORMES. 275 and extend more than halfway down the sternal segment of the 4tli rib as in Scythrops. The sternal ribs of Coua reynaudi are relatively longer than in other Cuckoos, and this fact appears to be correlated with degenerate powers of flight. As a result of the lengthened rib-segments and a change in the angle formed by the corpus sterni and the vertebral column, to be discussed presently, the form of the thoracic cavity differs conspicuously from that of other Cuckoos and recalls features characteristic of " Rat it a?." These peculiar features are wanting in Coua ccemdea. The cervical ribs of the Musophagi closely resemble those of the Cuculi, taking the form of broad pleurapophyseal lamellae. From the 5th to the 8th these lamellae are fenestrated as in the Cuckoos, so that the lamella appears to be joined to the centrum by a slender bar of bone, the bar extending to the level of the base of the postzygapophysis. There are six pairs of thoracic ribs, five of which articulate with the sternum. The 6th pair are long, but have lost connection with their sternal segments, which remain as small spicules anchylosed with the sternal segment of the 5th pair. In the Cuckoos, it will be remembered, there are only 5 pairs of thoracic ribs, the 5th pair of which, like the 6th in the Musophagi, fails to reach the sternum. v i. T he S t ern um a n d S h o u ld er - g ir d l e . The sternum of the Cuculiformes recalls that of the Coraciae rather than that of any other group. The sternum of the Cuculi may be readily distinguished from that of the Musophagi in that it is relatively conspicuously shorter, being indeed in many cases as broad as long. Moreover it appears to be undergoing yet further reduction. The form of the posterior margin of the corpus sterni and the relative development of the spina externa present considerable variations. With regard to the posterior margin of the sternum, this appears to be entire in Cacomantis only. Two extremely thin oval areas indicate, however, the presence originally of a pair of posterior lateral processes, the space between which and the median border of the metasternum has been filled up. In Scythrops, Cuculus, Coccystes, Guira, Centropus, and Croto-phaga the posterior border is interrupted by a single pair of notches, the outside of the notch being bounded by the posterior lateral process. All the other genera appear to have a doubly notched sternum, but the character of the notches varies considerably. In Piaya, for example, the posterior lateral process arises directly behind the articulation of the 4th rib, and extends outwards and backwards to terminate in a spatulate extremity some distance cephalad of a line passing across the free border of the metasternum. Between this process and the metasternum is a straight jwocessus intermedins. |