92 FOSSIL MAMMALIA OF THE 



incisors extends half waj' toward their base. The ridges and grooved trenchant premolars 

 Fig. 13. 



Fig. 14. 



Bettongia penicillata (Gould), mandible 

 and teeth, outer side, uat. size. 



Hypsiprymnus minor, mandible and teeth, 

 inner side, nat. size. 



occupy rather more than one sixth of the dental series. They are followed by four molars 

 resembling those of I{j/jmjjri/M//us, and equally adapted for vegetable diet. 



" The premolar in //. Gilberfii is but little longer than the foremost true molar, 

 whilst in H. murinHs it is equal in length to the first true molar tooth added to that of 

 the second." — Water house, op. cit., p. 230. 



In all Marsupial Pocphaga the mandible presents corresponding modifications for the 

 movements required in the cropping and mastication of vegetable food. The major part 

 of the condyle is horizontal ; and extended transversely, flattened, or with a feeble con- 

 vexity from before backward, which is its least diameter (PI. IV, fig. 10, c). The 

 condyle (figs. 13, 14, c) is raised above the level of the grinding teeth, about equidistant 

 from the inflected angle below (ib. a) and the summit of the coronoid process above. This 

 process (ib. b) is relatively narrow, short, and much inclined backward. The vertical line 

 from its summit to the condyle is less than half the same diameter of that part of the 

 ranuis. A wide vacuity at the fore and outer part of the ' ascending branch ' receives 

 the dentary canal from the inner surface, and is continued forward into the substance of 

 the horizontal ramus. 



In the absence of this external vacuity or perforation, and in the presence of every 

 character of jaw and teeth showing adaptation for animal diet, Plagiaulax (fig. 12) 

 differs from the Potoroos {Hypsipryvinus), Kangaroos {Macropvs), and every other known 

 recent or extinct form of poephagous Marsujnalia. 



We should have no ground for surprise if, in the long ages since the diprotodont 

 condition was first manifested, forms now exempUfying it had departed too far from the 

 primitive type to be zoologically associated therewith more nearly than as Marsupials with 

 lower incisors limited to a single pair. If we ask : — where is the living Marsupial that 

 retains the typical number of premolars with a reduction of the true molars to two and of 

 the incisor to one, on each side of the lower jaw ? — the answer is ' Nowhere.' 



In the case of the AmpJiithcr'uim or of the Spalacotheriuni , &c., with the excessive 

 number of molars, ]\Ir. "\A'aterhouse's discovery of Jli/rmecobius indicated the ]\Iarsupial 



