256 MAMMALIA. 



marsupials ; though some may perhaps belong to the Eutherian 

 order of Insectivora, while the few " Diprotodonts," as already 

 mentioned (p. 248), are more likely to have been related to the 

 monotremes. Such remains are known only from two horizons 

 in Britain, namely, from the Stonesfield Slate (Lower Jurassic) 

 of Oxfordshire and the Middle Purbeck Beds (Upper Jurassic) 

 of Dorsetshire ; also only from two horizons in North America, 

 namely, from the Upper Jurassic of Wyoming and the Laramie 

 Formation (Upper Cretaceous) of Wyoming and Dakota. Two 

 forms of mandible commonly placed here, Dromatherium and 

 Microconodon, are indeed as old as the Trias ; but these differ 

 so much from those of later date that their systematic position 

 is very uncertain, and they may belong to Anomodont reptiles 

 (p. 144). 



Nat. size. 



FIG. 150. 



Phascolotherium bucklandi; left mandibular ramus, inner aspect, three times 

 nat. size. L. Jurassic (Stonesfield Slate) ; Oxford, c, position of canine; 

 i, extent of incisor series; m, extent of molar series; pm., extent of the two 

 premolars. (After Goodrich.) 



The Jurassic and Cretaceous jaws are placed among the 

 polyprotodont marsupials because they exhibit so much re- 

 semblance to those of the existing Myrmecobius of Australia, 

 while many of them are proved to have been characterized by 

 the inflected angle. Phascolotherium and Amphitherium are the 

 best-known examples from the Stonesfield Slate, in which a 

 detached mammalian femur and a humerus have also been 

 identified ; Triconodon and Spalacotherium may be mentioned 

 from the Purbeck Beds, in which a few additional limb-bones 



