560 MANUAL OF ZOOLOGY. 



existing Mammals would seem to be the Marsupial and insec- 

 tivorous Myrmecobius, or Banded Ant-eater of Australia. As 

 only the teeth, however, of Microlestes have hitherto been dis- 

 covered, it is impossible to decide positively whether this 

 primeval Mammal was Marsupial or Placental. 



The next traces of Mammals occur in the Stonesfield Slate 

 (Lower Oolites), and here four species, all of small size, are 

 known to occur. Most of these were Marsupial, but it is 

 possible that one was placental. They form the genera Amphi- 

 lestes, Amphitherium, Phascolotherium, and Stereognathus. After 

 the Stonesfield Slate another interval succeeds, in which no 

 Mammalian remains have hitherto been found ; but in the fresh- 

 water formation of the Middle Purbeck, at the top, namely, of 

 the Oolitic series, as many as fourteen small Mammals have been 

 discovered. These constitute the genera Plagiaitlax, Trico- 

 nodon, and Galestes. Another gap then follows, no Mammal 

 having hitherto been discovered in any portion of the Cre- 

 taceous series (with doubtful exceptions). 



Leaving the Mesozoic and entering upon the Kainozoic 

 period, remains of Mammals are never absent from any of the 

 geological formations. From the base of the Eocene Rocks 

 up to the present day remains of Mammals invariably occur, 

 constantly increasing in number and importance, till we arrive 

 at the fauna now in existence upon the globe. 



The number of known fossil Mammals is so great, and they 

 exhibit so many peculiarities and divergences from existing 

 forms, that it will be impossible here to do more than simply 

 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 II. 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 



