OCR Text |
Show 646 ON THE ANATOMY OF THE JAQANAS. [May I 7, anteriorly. The postacetabular ridge has hardly any median projection ; and the pelvis is widest, dorsally, just behind the antitro-chanters. The plates of bone between the ischia and sacrum are narrower, and the posterior part of the renal fossae less well developed, and more open, in consequence. In all these points these forms thus approach the Limicoline birds. There is one other point of interest in the osteology of the Parridae. This is the extraordinary form assumed by the radius in some of the genera. In birds, as a rule, the ulna is a stouter bone than the radius, this last being almost universally a slender cylindrical bone. In Metopidius africanus, as already noticed by M . A. Milne-Edwards1, as well as in M. albinucha and in M. indicus (as I have been able to ascertain by extracting the wing-bones from a skin), the radius presents the form shown in the drawing (fig. 3), being dilated and Fig. 3. Wing-bones of Metopidius albinucha, to show the peculiarly modified radius; natural size. flattened into a subtriangular lamellar-like expansion for its distal half. Its superior surface is slightly grooved posteriorly for the tendon of the extensor metacarpi radialis longior muscle. This dilated portion forms the margin of the patagial space for its distal portion. A considerable portion of the marginal tendon of the tensor patagii longus is inserted into the radius at the angle of the bone ; the main tendon, however, continues in a groove on the inferior aspect of that bone, a little behind the border, to its ordinary insertion. About half of the peculiar flattened radius is left bare of muscle above, the extensor metacarpi, as already stated, playing over its lower half. Below, the flattened area is largely covered by the fibres of the pronator radii superficialis, which extend up nearly to the margin of the bone; below this is the pronator radii profundus, which likewise has an extensive insertion into the lower part of the bone. The margin of the bone, where it is superficial, is slightly roughened; and no doubt the peculiar form of radius is associated with the quarrelsome habits of these birds, this dilated and somewhat scimitar-shaped bone being probably capable of inflicting a very severe downward blow. In Parra jacana and P. gymnostoma the radius presents the ordinary form; and the same is the case in Hydrophasianus chirurgus. In these two genera, it is to be observed, the metacarpal " spur " 1 ' Oiseaux Fossiles,' ii. p. 134. |