VERTEBRATA : MAMMALIA. 653 



hairs in some part or other of the skin of all Mammals is 

 constituted by the Cetacea, the great majority of which are 

 without hairs in the adult state. Some, however, occasionally 

 possess a few bristles in the neighbourhood of the mouth even 

 when fully grown. And the Dolphins, which are totally hair- 

 less when adult, exhibit tufts of hair on the muzzle in the 

 foetal state. 



The claws, hoofs, and nails of Mammals, and the horny 

 sheaths of the horns of the Cavicorn Ruminants, are also of 

 the nature of epidermic growths. 



Lastly, the Armadillos are remarkable for having plates of 

 bone developed in the dermis over a greater or smaller por- 

 tion of the surface. 



DISTRIBUTION OF MAMMALIA IN TIME. As a matter of 

 course, the remains of Mammals are scanty, and occupy but 

 a small space in the geological record, since the greater 

 number of the Mammalia are terrestrial, and the greater num- 

 ber 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 Vertebrata. 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 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 



