426 PKOCEEDINGS OF THE eEOLOGICAL SOCIETY. [Jline 18, 



outer, is convex and triangular, being narrower towards the summit 

 of the crown. It passes gradually into the anterior side, which is also 

 triangular, but stHl narrower. From the vertical depressions on the 

 outer surface two grooves extend inwards on to the crown, which is 

 thus divided by two transverse valleys separated by elevations. Of 

 these, the two posterior, broad and ridge-like, join internally to form 

 the inner surface of the tooth ; while the anterior, which has more the 

 form of a cone than that of a crest, is not more than half as broad as 

 the others, and terminates, internally, in a smoothly rounded convex 

 pillar, which remains distinct to the base of the crown. From its 

 anterior surface a ridge springs, which, gradually decreasing in 

 height, skirts its base and then ascends, upon the inner part of the 

 middle ridge, to form the anterior boundary of the inner face of the 

 tooth. The posterior basal ridge is well marked and concave up- 

 wards; its inner and outer ends, as it were, ascending upon the 

 postero-extemal and postero-intemal angles of the tooth. The 

 anterior, or mammUlary, elevation is not at all worn in either 

 tooth. The middle and posterior ridges are slightly worn, so as to 

 give rise to two elongated facets, each not more than one- sixth of an 

 inch wide, and passing into one another internally, being separated 

 only by the posterior groove, which dilates somewhat suddenly at its 

 inner end (fig. 2). 



The premolar of No. 2 is constructed upon precisely the same 

 general plan as that of No. 1, but differs in several details. Thus, 

 it is slightly smaller, and the antero-internal ridge which skirts the 

 base of the mammilla has a somewhat different form. But the 

 most marked difference is offered by the outer surface of the tooth 

 (fig. 4), which presents not merely three smoothly convex surfaces, as 

 in the other specimen, but exhibits three well-defined vertical ridges, 

 connected by prominent, curved, basal elevations. The premolar of 

 this specimen is altogether somewhat smaller than that of the other. 



That both these specimens are specifically distinct from the only 

 species of Bij^rotodon known at present, viz. J), australis, appears 

 likely, at first sight, from a comparison of the dimensions of the 

 corresponding teeth. 



In the maxilla of Dijprotodon austraUs (British Museum, No. 

 32848), to which I have already referred, the socket of the premolar 

 and the first and second molars occupy a space of 4| inches in the 

 alveolar margin of the maxilla : in No. 2 the same teeth occupy 

 only about 3| inches. The measurements of the individual teeth, 

 given in eighths of an inch in the following table, present a nearly 

 similar ratio. 



D. australis (B,M.). 

 Breadth. Length. 

 5 8* 



13 12 



16 15 



17 20 



* These are measurements of the alveolus and its contained fang. The crown 

 of the tooth was doubtless much larger in each dimension. 



Premolar 



First molar 



No. 2. 

 Breadth. Length. 



6^ n 



9i 10 



No. 1. 

 Breadth. Length. 

 7 8 

 12 10 



Second molar . 

 Fourth molar . 



..... 1\\ 12 

 13 16 





