OCR Text |
Show 1893.] PROP. G. B. H O W E S ON HAPALE JACCHTJS. 169 from the ventral aspect owing to the approximation in the ventral middle line of their parent (seventh pair of) ribs, in the manner customary for other Mammals. Variability in ossification of the fully formed mesosternal segments of Mammalia, such as may bring about irregularities of the sternebra? with a more or less alternate disposition of the lower ends of the opposite sternal ribs, is no uncommon phenomenon, especially among the Primates ; but it rarely affects the presternum1. In the case under consideration, the base of that segment was correspondingly asymmetrical, repeating, in relation to the first mesosternal segment, Fig. 1. Fig. 2. Sternum of Hapalc jacchus, viewed from tbe inner (dorsal) aspect. 1. Normal. 2. Abnormal. Adults, nat. size. st'. Presternum, st". Mesosternum. st'". Post-or xiphisternum. the relations of this to its fellow next in order of succession behind. The mesosternum, while retaining, in respect to the independence of its constituents, a persistently embryonic condition, bad so far increased in width during growth that the transverse diameter of each succeeding sternebra exceeded, at its narrowest point, that of the expanded base of its normal counterpart. Comparison with the normal adult (fig. 1) showed that the sternum, as a whole, had undergone an increase in breadth proportionate to reduction in length, due allowance being made for the occasional presence of but four distinct mesosternal segments2. When viewed from the inner aspect (fig. 2), the extreme regularity of 1 See tbe skeletons of the Gibbon in the Museum of Natural History, and of the Orang in the Teaching Collection E. College of Science, South Kensington. 2 Mivart, P. Z. S. 1805, p. 508. |