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Show 784 PROF. BALDWIN SPENCER ON A [Nov. 20, In regard to their dentition as well as to the nature of the foetal membranes, Wilson and Hill' have come to the conclusion that there is very clear evidence of retrogression within the marsupial order, and it appears to be very probable that this retrogression has affected the cranial development. The Lower Jaw.-Only a fragment of one ramus of the lower jaw (figs. 6 & 7, P L L.) is preserved together with a broken-off coronoid process (fig, 8), and, just as in the case of the upper jaw, no teeth are visible. The angle of tbe jaw has the inward bending so as to form, as seen from below (fig. 7), a flat surface of considerable extent, which evidently formed, as in Potorous and Phalanger, the floor of a deep and fairly wide depression on the inside of tbe ascending ramus, which is completely broken away. It differs, however, from that of the Phalangeridae in being of much greater proportionate length and in running backwards into a long and markedly upturned process, more like that in Perameles than in any other form. It will be noticed that there is on the under surface a well-marked ridge which accentuates the convexity of the outline when seen from the side. The structure of tbe jaw, while distinctly marsupial, is in certain respects unlike that of any known form. The inflected angle, which is in fact longer than is represented in the figure (a broken off portion 8 m m . in length not being represented), is quite unlike that of any existing species both in regard to its length and upward curvature. It must also have been of considerable vertical height, as there were no traces left of even the lower parts of the sockets ; and either there were no largely developed incisors, or else tbe symphysis was a remarkably long one, as there is not a trace of any sockets at the anterior end. The ventral curvature and sharply marked ridge are again characteristic features, while further still there is a curious and inexplicable feature in regard to the ramus, consisting in the presence of a deep depression across the upper surface, which must apparently have been situated in tbe area occupied by the molar series. It extends in a slanting direction from side to side across the jaw, and its smooth, rounded floor-surface and margins show clearly that it is not a mere break in tbe jaw. With this is associated a swelling on the internal face, which is well seen in the view from below (fig. 7). What can have been the meaning of this it is impossible to state *, as it is apparently unlike anything known in any mammalian jaw, while its definite outline and the normal condition of the surface of the bone appear to negative the idea that it is a pathological feature. The dental foramen is not present, having evidently been placed farther back than the most posterior limit of the preserved portion of the ramus. The mylo-hyoid groove, if present, is only very faintly marked, contrasting in this respect strongly with earlier marsupials, 1 J. P. Hill, " The Placentation of Perameles," Q. J. M. S. vol. xl. p. 435. 2 The only suggestion which can be made is that possibly it m a y have been associated with the presence of a large sectorial tooth in the molar or premolar series such as is developed in Abder/tes, |