OCR Text |
Show 276 MR. W. T. PYCRAFT ON THE [Mar. 17, which, like the external process, fails to reach the level of the meta-sternal border. In Rhinococcyx, Rhamphococcyx, and Taccocoua the posterior lateral process arises, as in Piaya, immediately behind the articulation for the 4th rib, but it forms a wider angle with the long axis of the sternum, and carries with it the intermediate process, so that it looks like a branch thereof. In Coua and Geococcyx the posterior lateral process leaves the sternal plate further back than in the forms just described. In other respects these two sterna are quite distinct. In Coua ccerulea the posterior lateral process is very broad, and extends backwards to the level of the posterior border of the metasternum, whilst the intermediate process is short and normally placed. Coua reynaudi, with an obviously degenerate sternum, differs from C. ccerulea in that the posterior lateral process is slender, and the intermediate process shows a tendency to fuse with the base of the posterior lateral. In Geococcyx the posterior lateral process is long and slender, but fails to reach the level of the free border of the metasternum ; whilst the intermediate process lies nearer the posterior lateral than the metasternum. Further, the sternum is peculiar in the great depth of the fissure on either side of the metasternum, giving this a long and narrow shape. A conspicuous pointed spina externa is found in Guira, Cuculus, Geococcyx, and Scythrops. It is vestigial in Centropus. A small spina interna occurs in Taccocoua. In Rhamphococcyx the spina externa and interna are both present, and fused to form a moderately large spina communis. In Piaya, Rhinococcyx, and Coua there is a vestigial spina communis. In Cacomantis the two spines remain distinct, but are extremely reduced. Both spina externa and interna are wanting in Crotophaga. The carina sterni is relatively largest in Cuculus (text-fig. 43, p. 277), Cacomantis, and Scythrops. Its free ventral border is strongly arched, and its anterior border is deeply concave. The antero-ventral angle of the keel affords a firm articulation for the furcula. In Coua, Rhamphococcyx, Crotophaga, and Piaya, the anterior border of the keel is very deeply emarginate, causing the free border of the keel to project forward in the form of a long style. With the dorsal surface of the style the clavicle articulates by means of a long hypocleideum. Apparently the length of the hypocleideum is directly correlated with the retreat of the carina caudad. In the Galli this is well seen, the climax being attained in Opisthocomus. The depth of the carina is extremely reduced in Coua reynaudi (text-fig. 44, p. 277), so much so indeed that the power of flight in this species must be extremely limited. In other Cuckoos the depth of the carina is nearly or quite equal to half the breadth of the corpus sterni; in the species in question the greatest depth of the keel is barely one-fourth the breadth of the sternal plate. Correlated with the reduction in the keel and the consequent loss of flight, is a marked change in the position of the sternum, which |