THE BRITISH MAMMALS PAST AND PRESENT. 8l 



Microlestes from the Rhaetic series, which comes, as we have said, 

 between the Trias and Lias, and consequently almost at the base of 

 the Secondary rocks. These are known by their teeth, one species, 

 M. moorei, being from near Frome, the other, M. rhceticus, from 

 Watchet. They are generally considered to be Marsupials, 

 although, like the Wombats, the teeth have no continuations of the 

 dentinal tubes traversing the enamel. It is inconceivable that they 

 were the first mammals to die in this country, or even in what is 

 now Somersetshire, but it is remarkable that no further traces of 

 mammalian life have yet been discovered until we reach the Stones- 

 field Slate, none in the Lias, none in the Inferior Oolite, none until 

 we arrive at the thin-bedded limestones at the base of the Great 

 Oolite, which are called slates fsom being locally used for roofing 

 purposes, and are unlike slates in every other particular. 



These thin limestones have yielded five genera, all of them 

 Marsupials, Phascolotherium, Amphilestes, Amphitheriutn, and Amphi- 

 tylus, all belonging to the same family, and Stereognathus, of a family 

 by itself, whose position among the Marsupials is by some con- 

 sidered to be doubtful, as in a less degree is that of Amphit'herium, 

 which has a larger number of teeth than any living member of the 

 order. 



No mammals have been found in any of the overlying Jurassic 

 beds until we reach the Purbecks at the top of the series, which 

 differ so markedly in character from those below. In this group, 

 which is largely composed of freshwater deposits, and contains 

 among its numerous fossils a few genera of lake and river shells that 

 still exist, a series of estuarine and shallow marine strata is placed 

 between an upper and lower set of freshwater origin, and near the 

 base of these middle beds a band some five inches thick in Durdle- 

 stone Bay has yielded no less than eleven genera of Marsupials, 

 none larger than a rat. 



These consist of four species of Plagiaulax, one species of 

 Bolodon, three species of Triconodon, four species of Amblothtrium, 

 two species of Achyrodon, one species of Kurtodon, one of Peramus, 

 two species of Stylodon, one species of Leptocladus, two species of 

 Spalacotherium, and one of Peralestes, making twenty-two species 

 altogether, really a remarkable assemblage for so limited a field. 



In the superposed secondary rocks of this country no mammalian 

 fossils have been recorded, or, at least, nothing structural to which 

 a name can be given, but in North America the upper beds of the 

 cretaceous series have yielded a number of small mammals closely 

 allied to the forms from the Rhaetics and Purbecks above mentioned, 

 thus indicating that the chain of mammalian life was unbroken, 

 although the links were preserved in a different area. That the 

 mammals of the land should be found in a sea deposit like the chalk 

 is hardly to be expected, but it is not improbable that we may in 

 time light upon their representatives in the fluviatile and littoral 

 deposits of our cretaceous group, such as the Wealden and Lower 

 Greensand. 



With the incoming of the Tertiary period we have a different 

 state of things. In the Lower Eocene, mostly in the London Clay, 

 we have discovered our first carnivore, Argillotherium toliapicum, 



