INTRODUCTION. 61 



In analyzing the Marsupiata, with a view to deter- 

 mine with which group to commence the order, I 

 was anxious to select a genus, the species of which 

 would exhibit the most perfect dentition, and at the 

 same time present other characters which are com- 

 mon in the species of that order. These characters 

 I found combined, in the most perfect manner, in 

 the true Opossums (Didelphys) they possess the 

 greatest number of teeth, (if we except the extra- 

 ordinary abnormal dentition of Myrmecobius,*) a well 

 developed thumb to the posterior extremities, their 

 limbs are fitted either for climbing or walking, and 

 their tail is muscular, and capable of being used as 

 an organ of prehension.* In addition to these nor- 

 mal characters, I learn, from Professor Owen's paper, 

 that nearly the whole of the Marsupiata are provided 

 with an intestinum caecum, and in this respect the 

 true Opossums may still be regarded as affording a 

 good typical representation of the order. I may, 

 moreover, observe that, as a general rule, animals of 

 omnivorous or fugivorous diet are higher, in their 

 own order > in the grade of organization, than either 

 the carnivorous, insectivorous, or herbivorous species. 

 Again, it is well known that the Opossums t are 

 sometimes destitute of the abdominal pouch, J and I 



* The enormously strong, muscular tail of the Kangaroos, 

 can only be regarded as presenting a slight modification of the 

 prehensible character of tail so common in the Marsupiata. 



+ I always use this term in its restricted sense that is, for 

 the genus Didelphys. 



$ Are the young of those Opossums, in which the pouch is 

 rudimentary, more perfectly developed at their birth, than the 

 young of other Marsupials ? 



