GENERAL CHARACTERS OF THE MAMMALIA. 283 



in the deciduous series, only existing once, and not being 

 replaced by successors. 



All these four kinds of teeth are not necessarily present in 

 all Mammals, and, as will be afterwards seen, the characters 

 of the teeth are amongst the most important of the distinc- 

 tions by which the Mammalian orders are separated from 

 one another. The variations which exist in the number of 

 teeth in different Mammals are usually expressed by a 

 " dental formula," which presents the " dentition " of both 

 jaws in a condensed and easily-recognised form. 



According to Owen, the typical permanent dentition of a 

 diphyodont Mammal would be expressed by the following 

 formula : — 



. 3—3 1—1 4—4 3—3 , , 



3—3 1—1 4—4 3—3 



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

 the letters — incisors i, canines c, premolars j^5«2., 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 en- 

 tire sub-kingdom of the Vertebrata ; and therefore, in obe- 

 dience 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 cari'ied 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 



