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Show 160 MR. R. H. BURNE ON THE [Feb. 17, Ruge has pointed out that the xiphisternum (processus ensiformis) of M a n owes its origin to the eighth and ninth pairs of ribs ; for he says, in the course of the paper above referred to, "das achte und neunte Rippenpaar vom Sternum sich loslosend ihre medialen Produkte noch im Processus ensiformis wiedererkennen lassen." The Leporine sternum usually consists of four segments, exclusive of the omosternal and xiphisternal ones, and it therefore follows that that pair of ribs whicb stand related and give origin to the last prexiphisternal sternebra is the sixth pair; and as this is so the xiphisternum, on analogy to the human subject, might be expected to arise from the seventh and eighth ribs, one or both. The seventh pair of ribs ultimately break loose and approximate in the ventral middle line. Not unfrequently, in both Rabbits and Hares, there is an extra sternebra intercalated behind the sixth ( = fourth sternebra as ordinarily enumerated), and the occurrence of this structure (figs. A-F, p. 161) has not previously been recorded. This extra sternebra is invariably displaced dorsally; the lower ends of the seventh pair of ribs pass over its ventral surface and meet in the middle line, repeating as it were, in regard to it, those relationships which they more normally bear to the head of the xiphisternum. The sternebra in question always lies behind the sixth pair of ribs ; these are attached to the synchondrosis between the sixth and the extra sternebra (VII), wherefore the latter would appear to be derived from the seventh pair of ribs. This seventh sternebra is very variable in size, presumably as the result of variation in degree of absorption. It is probably not uncommon, for of thirteen fully ossified Leporine sterna, which I have examined, six possessed it in some form, and although in one case somewhat minute, it was yet distinctly recognizable. Figs. A - G represent a series of fully formed sterna. In figs. A, B, C, D, E the extra sternebra (S. VII) will be seen to be represented more or less by bone; but in fig. F it is only present in cartilage, and in fig. G it is not present at all. In rare cases the two portions of the primitive cartilages from which this sternebra is derived may be found still more or less ununited, a distinct groove being recognizable upon its dorsal surface (fig. F). I have found this to be so in animals possessed of an otherwise fully formed and ossified sternum. Be the individual conditions what they may, the seventh pair of ribs invariably meet in the ventral middle line, and are in no way attached at the sides of their intersternebra as are the ribs in front of them. Turning now to the development of the seventh or "extra" sternebra. The earliest stage in its formation which I have observed was in an embryo Rabbit 3 cm. long. At this age (fig. I) the median cartilages of the sternum had only fused at their anterior ends ; the first seven pairs of ribs were joined to them, but the rest floated free. The costal segments related to each pair of ribs had manifestly been formed each in the same way, the only |