MONOTEEMATA AND MARSUPIALIA. 



293 



case with the ordmary Diprotodont Marsupials ; and the 

 molars are very similar in type to those of Microlestcs. 

 The remaining three genera of the Purbeck beds — -viz., 

 Sjxdacotherium, Triconodon (fig. 604, 2), and Galestes — ap- 



«i**6ajStottti 





Fig. 604. — Oolitic Mammals, natvtral size. 1, Lower jaw and teeth of Phascolothermm ,- 

 2, of Triconodon ; 3, ot Amphitherium ; i, of Plagiaiilax. 



Fig. 605. — A, Right ramus of lower jaw of Plagimilax minor — Jurassic, enlarged four 

 diameters ; b, Third praemolar of Plagiaulax Becldesii, enlarged five and a half times, and 

 sliowing the diagonal grooves on the exterior of the crown. (After Owen.) 



pear to have been certainly insectivorous, and find tlieir 

 nearest living allies in the Australian Phalangers and the 

 American Opossums. 



In the North American area, the earliest known remains 

 of Marsupials have been obtained by Professor Marsh from 

 beds of Upper Jurassic age. These indicate the existence of 

 a small Opossum {Dryolestcs priscus), belonging to the family 

 of the Bidcljyhidcc, and thus of special interest as showing 

 that this peculiarly American type was differentiated at a 

 date so comparatively early, within the same geographical 

 region as that in which it now occurs. 



Coming next to the Tertiary deposits, we hnd that few 

 traces of this order have been as yet discovered. In the 

 Eocene Tertiary of the Paris basin, we meet with true Opos- 

 sums (Didelphidoi), closely resembling the existing American 

 species of Didd^ohys, though the distinct genus Peratherium 



