GENERAL CHARACTERS OF THE MAMMALIA. 405 



According to Owen, the typical permanent dentition of a 

 diphyodont Mammal would be expressed by the following 

 formula : 



33 i i 44 33 



The four kinds of teeth are indicated in such a formula by the 

 letters incisors i, canines c, praemolars pm, molars m. The 

 numbers in the upper line indicate the teeth in the upper jaw, 

 those in the lower line stand for those in the lower jaw; and 

 the number of teeth on each side of the jaw is indicated by 

 the short dashes between the figures. 



As regards their general distribution 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 

 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 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 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 



