DISTRIBUTION OF MAMMALIA TN TIME. 609 



point out the leading facts known as to the distribution of each 

 order of Mammals in past time. 



Order I. Monotremata. — The Monotremes are not known to 

 be represented at all in past time ; and this need not excite 

 any surprise, seeing that the order is represented at the present 

 day by no more than two genera, both confined to a single 

 geographical region. Upon theoretical grounds, however, it 

 may be expected that we shall ultimately discover that the 

 antiquity of the order Monotremata is extremely high. 



Order IT. Marsupialia.— This is probably the oldest of the 

 Mammalian orders ; but owing to the detached and fragmen- 

 tary condition of almost all Mammalian remains — consisting 

 mostly of the ramus of the lower jaw, or of separate teeth— it 

 is not possible to state this with absolute certainty. The 

 Microlestes of the Trias, the oldest, or nearly the oldest, of the 

 Mammals, was probably a Marsupial; but the evidence upon 

 this point is not conclusive. In the Triassic rocks of America, 

 also, perhaps at a lower horizon than that at -which Microlestes 

 occurs in Europe, has been found the jaw of a small Mammal, 

 which is probably Marsupial, and has been named Droma- 

 therium (fig. 236). 



Fig. 236. — Lower jaw of DromatJierium sylvestre (after Emmons). From rock!^ 

 supposed to be of Triassic age, in North Carolina. 



In the next mammaliferous horizon, however — namely, that 

 of the Stonesfield Slate in the Lower Oolites — there is no doubt 

 but that some of the Mammalian remains, if not all, belong to 

 small Marsupials (fig. 237). From this horizon the two genera 

 Phascolotherium and Amphitherium, are almost certainly refer- 

 able to the Marsupialia; the latter seeming to be most nearly 

 related to the living Myr7necobius, whilst the former finds its 

 nearest living ally in the Opossums of America. The Stereog- 

 nathus of the Stonesfield Slate is in a doubtful position. It 

 may have been Marsupial; but, upon the whole, Professor Owen 

 is inclined to believe that it was placental, hoofed, and her- 

 bivorous. 



With the occurrence of small Marsupials in England within 

 the Oohtic period, it is interesting to notice how the fauna of 

 that time approached in other respects to that now inhabiting 

 Australia. At the present day, Australia is almost wholly 



