Mr. G. R. Waterhouse on the Classification of Mammalia. 407 



species framed upon some general and particular model ? Cer- 

 tainly it may be said with respect to the Water-rat {Arvicola am- 

 phibia) , that it is framed on the Vertebrate model ; on the Mam- 

 malian type of that model ; on the Rodent type of the Mam- 

 malia ; and it is equally clear to my senses that it possesses the 

 same general structure of skull, combined with the anchylosed 

 fibula to the tibia, &c., which characterize the Murine family of 

 the Rodent order ; but, beyond this, it exhibits a modification in 

 the structure of the teeth in which it agrees with numerous other 

 species of the family mentioned, and which are classed under the 

 generic title Arvicola. So that in one sense the Water-rat may 

 be said to be essentially framed upon more than one model, but, 

 from the lowest to the highest of the divisions mentioned, each 

 model is a modification of the type of the division which pre- 

 cedes it ; and the case might be therefore symbolically represented 

 by concentric circles of difibrent sizes, the largest of which would 

 typify the Vertebrataj and the smallest the genus Arvicola and so 

 on. It does not appear that the Water-rat is framed upon two 

 or more types of equal rank, and I strongly incline to the belief, 

 that what is true of one species, as regards the point under con- 

 sideration, is true of all. 



There is one other point relating to the genera introduced in 

 the table to which I wish to call attention, viz. that it often hap- 

 pens that those species of one order which approach most nearly 

 to other adjoining orders, are not met, as it were, by a corre- 

 sponding approach in those adjoining orders. Each order may. 

 throw out rays (to speak figuratively) to other orders, but the 

 rays are seldom in the same direction. I have noticed one case 

 illustrative of this point, that of the Wombat and Lagostomus : 

 many might be adduced. Among the Carnivora, the genus My- 

 daus in general appearance and in its insectivorous diet resembles 

 the species of the order Insectivora ; but it differs widely in its 

 dentition, having but one true molar to each side of each jaw, as 

 in others of the group to which it belongs. On the other side 

 we find a considerable approach evinced in the genus Gymnura 

 (one of the Insectivora) to the Carnivorous order*, displayed in 

 the general form of the skull, in the presence of six incisors (a 

 number unusual in the Insectivora), and weU-developed canines. 

 Here I can only perceive an approach, on the one hand, of one of 

 the Insectivora to the order Carnivora, and on the other, one of 

 the Carnivora approximating to the Insectivora. But the two 

 animals mentioned do not approach towards each other in corre- 

 sponding modifications of structure, for the Gymnura would bear 

 a closer comparison with some of the small Ursidce, where the 



* This animal in fact was originally described as a Viverra. 



