784 PROP. BALDWIN SPENCER ON A [Nov. 20, 



In regard to their dentition as well as to the nature of the total 

 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, PL 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 the 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 the 

 ascending ramus, which is completely broken away. It differs, 

 however, from that of the Phalangeridac 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 the 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 mm. 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 the 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 the 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 the 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 2 , 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 Plaeentation of Perameles" Q. J. M. S. vol. xl. p. 435. 



2 The only suggestion which can be made is that possibly it may have been 

 associated with the presence of a large sectorial tooth in the molar or premolar 

 series such as is developed in Ahderites. 



