254 HISTORICAL PALAZONTOLOGY. 
the latter is supposed to find its nearest living ally in the 
Opossums (Dide/phys) of America. Lastly, the ‘Stereognathus 
Fig. 183.—Lower jaw of Asphitherium (Thylacotheriune) Prevostit. 
Stonesfield Slate (Great Oolite.) 
of the Stonesfield Slate is in a dubious position. It may have 
been a Marsupial; but, upon the whole, Professor Owen is 
inclined to believe that it must have been a hoofed and her- 
bivorous Quadruped belonging to the series of the higher Mam- 
mals (Placentalia). In the Middle Purbeck beds, near to the 
close of the Oolitic period, we have also evidence of the exist- 
ence of a number of small Mammals, all of which are probably 
Marsupials. Fourteen species are knewn, all of small size, 
the largest being no bigger than a Polecat or Hedgehog. The 
genera to which these little quadrupeds have been referred are 
Plagiaulax, Spalacotherium, Triconodon, and Gatlestes. The 
first of these (fig. 184, 4) is believed by Professor Owen to 
Fig. 184. Oolitic Mammals.—1, Lower jaw and teeth of Phascolotherium, Stonesfield 
Slate ; 2, Lower jaw and teeth of ‘Amphither tun, Stonesfield Slate ; 3, Lower jaw and 
teeth of Triconodon, Purbeck beds ; 4, Lower jaw and teeth of Plagiaulax, Purbeck 
beds. All the figures are of the natural size. 
have been carnivorous in its habits; but other authorities 
maintain that it was most nearly allied to the living Kangaroo- 
rats (Hypsiprymnus) of Australia, and that it was essentially 
herbivorous. The remaining three genera appear to have 
been certainly insectivorous, and find their nearest living rep- 
resentatives in the Australian Phalangers and the American 
Opossums. 
Finally, it is interesting to notice in how many respects the 
