DISTRIBUTION OF MAMMALIA IN TIME. 639 



greater number of the stratified fossiliferous deposits are marine. 

 The Mammals, too, are the most highly organised of the entire 

 sub-kingdom of the Vertebrata; and therefore, in obedience to 

 the well-known law of succession, they ought to make their 

 appearance upon the globe at a later period than any of the 

 lower classes of the Vertebraia. Such, in point of fact, is to a 

 great extent the case ; and if the geological record were perfect, 

 the law would doubtless be carried out to its full extent. 



It is in the upper portion of the Triassic Rocks that is to 

 say, not long after the commencement of the Mesozoic or 

 Secondary epoch that Mammals for the first time make their 

 appearance ; three or four species being now known in a zone 

 of rocks which 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 described under the name of 

 Microlestes antiquus. The nearest ally of Microlestes amongst 

 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 Plagiaulax, 

 Spalacotherium, Triconodon, and Galestes. 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. 



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 



