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Show 4b'6 MR. P. E. BEDDARD ON INTERCENTRA IN BIRDS. [Ma 4, The hypocentra-as may be seen in various species with free intercentra-are intercentra which are attached to the summit of short outgrowths of the centra. Thus the coalesced hypapophysis is really intercentrum plus this median central outgrowth. In several cases where there are no free intercentra, but only the hypocentra, it would be difficult to assert that the latter are really, as has been stated, partly produced by the intercentral discs of bone. They appear to belong so entirely to the centrum. But that this is not the case can be readily seen from such examples as that shown in the drawing (woodcut, fig. 1), where the free intercentra gradually come to be more and more thoroughly articulated with the centrum behind. The intercentra when present as free structures appear invariably to increase in size from before backwards. They often begin as little more than mere granules of bone, which, as they are often easily detachable, m a y have been missed, though really present. They are especially loosely attached, and therefore liable to be lost, in the Auks. So far as can be judged from the facts which are briefly stated in the following pages, the existence of free intercentra is not universal but very general among birds. There are, however, groups, such as the Cuckoos and Columbae, in which these structures are not present as independent bonelets. There are also some birds in which they do not appear to exist at all; in which, indeed, there are no hypapophyses which may be presumed to be these structures, but this is rare. They are obviously most prevalent in water-birds, including, however, the land Limicolae, which are so nearly akin to the Gulls and Auks. They are rarest among arboreal birds, such as Parrots, Pigeons, Picarian and Passerine birds; but occur in the possibly archaic Opisthocomus. In the reduced tail of the Struthiones intercentra do not occur plentifully. Parker, as has been already mentioned, finds two in Apteryx. But Mivart1, in his elaborate description of the axial skeleton of the Struthiones, does not appear to have met with them. There does not appear to be a very definite connection between the presence of intercentra and the lowness of the place of the bird in the series; but, on the other hand, there does, with some exceptions, seem to be some relationship between the length of tbe tail and the existence of free intercentra. It is not known to what extent these structures existed in most of the important types of extinct birds; but Marsh found two free intercentra in Hesperornls2. ANSERES. In Biziura lobata (fig. 1) the intercentra are especially well developed. There is a small oval nodule 3 m m . long between the last sacral and the first free caudal, a rather larger one between this and the following vertebra, and in the next interval a large 1 " O n the Axial Skeleton of the Ostrich," Zool. Trans, viii.; and " O n the Axial Skeleton of the Struthionida.," ibid. x. 2 See his great work upon the " Odontornithes." Washington, 1880. |