1G8 THE KANGAROOS. 



would have taken place : on the side of the upper 

 iaw in which there exists six molars, the two fore- 

 most would have been thrust out by a false molar, 

 which is in an advanced state of development be- 

 neath them ; on the opposite side of the upper jaw, 

 hut one tooth would have been displaced, the other 

 having already fallen. Now, as the permanent, or true 

 molars, are successively developed one behind the 

 other, the new teeth thrust forward those which 

 precede them, and the foremost of the series, owing 

 to the pressure from behind, begins to decay, or to 

 be absorbed at the root and soon falls away from the 

 socket. Professor Owen examined a skull of a very 

 old M. major in which there were only two molars 

 on each side of each jaw, showing that this vis a tergo 

 does not stop even when the animal has attained ma- 

 turity. In the skulls of five or six other species of 

 Kangaroo which have come under my notice, I in- 

 variably found the number of teeth as stated at the 

 commencementof this account of the dental formula.* 



* Having ascertained the number of true molars in the Kan- 

 garoos, there is no difficulty in tracing the analogous teeth 

 throughout the remaining groups of Marsupials, and the result 

 is, that they have the same number, viz., four true molars, 

 whilst in the other orders of Mammals the highest number is 

 three at least I have only found a few apparent exceptions, 

 nnd these exist in groups, the dentition of which I have not 



yet had an opportunity of studying in two or three of the 



genera of Insectivora, there appears to be more than three true 

 molars : it must be borne in mind, however, that the false 

 molars often assume the common form of true molars, and 

 vice versa ; hence from a mere inspection of a skull it is often 

 impossible to distinguish their teeth. 



