284 MAMMALIA. 



a zone of rocks wliicli are placed at the summit of the Trias, 

 just where this formation begins to pass into the Lias. The 

 earliest of these — the oldest known of all the Mammals — 

 appears at the upper part of the Upper Trias (Keuper) and 

 also at its very summit (Penarth beds), and has been de- 

 scribed under the name of Microlcstcs antiquus. The nearest 

 ally of Microlestcs amongst existing Mammals would seem to 

 be the Marsupial and insectivorous Myrmecobius, or Banded 

 Ant - eater, of Australia. As only the teeth, however, of 

 Microlcstes have hitherto been discovered, it is impossible 

 to decide positively whether this primeval Mammal was 

 Marsupial or Placental. In the North American area, also, 

 the first traces of Mammals {Dromatherium) appear in the 

 Trias. 



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 pos- 

 sible that one was Placental. They form the genera Amjyhi- 

 lestes, Anvphithcrium, Phascolothermm, and Stereognatlms. 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 

 Plagiaulax, Spalacothervmn, Triconodon, and Galcstes. The 

 Upper Jurassic of North America has also yielded a small 

 Opossum. Another gap then follows, no Mammal having 

 hitherto been discovered in any portion of the Cretaceous 

 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 commonly 

 occur, constantly increasing in number and importance, till 

 we arrive at the fauna now in existence upon the globe. 



It should be noticed, further, that there are many points 

 in which it can be demonstrated that the earlier Tertiary 

 Mammals were of a more generalised type than those now 

 existing, and in particular groups a very well marked pro- 



