INTRODUCTION. 71 



amongst corresponding groups of the Placental Mam- 

 malia.* 



It must not, however, be inferred, from the ob- 

 servations just made, that the species of Marsupiata 

 resemble, in their dentition, those of the other orders 

 of Mammalia with which they have been associated. 



The Opossums have been arranged with the Insec- 

 tivora, and they have moreover been classed, as be- 

 fore stated, in conjunction with the Phalangers, with 

 the Qiuadrumana ; the Thylacinus and Dasyures, 

 have been arranged with the Carnivora, and the 

 Wombat with the Rodentia. But in all these cases 

 where Marsupial animals have been associated with 

 species of other orders, there exist essential differ- 

 ences in the dentition. 



The normal number of incisors in the Placental 

 series is six above and six below — throughout this 

 gi-eat section there are none in which I can find more 

 than this number — any decrease in the number is 

 comparatively unimportant, inasmuch as very nearly 

 allied species often differ in the number, and even in 

 the adult animal, there are not unfrequently less 

 incisors than in the young of the same species. Now, 

 in the Marsupiata, the highest number of incisors is 

 eighteen — ten above and eight below, t — hence when 

 compared with the Quadnima?ia, the Opossums pre- 



* Professor Owen " On the Structure of the Brain in Mar- 

 supial Animals," Philosophical Transactions, Part I. for 1837. 



+ In nearly all the Marsupial animals there are more in- 

 cisors in the upper jaw than the lower, whereas in other 

 orders of Quadrupeds, when a difference exist, the increase is 

 generally in the lower jaw. 



