or THE CLASS MAMMALIA. 13 



dont Monotremata, JBruta, and Cetacea, formed an equivalent group 

 with the diphyodont bulk of the Mammalia, or that the binary- 

 groups, defined by this single dental character, were natural ones. 



Nothing more than a passing allusion seems needed to the 

 system of classifying the Mammalia on the modifications of the 

 placenta, originally proposed by Sir Everard Home*, and since 

 reproduced and modified by a few other naturalists. The group, 

 e. g. associated by the character of the discoid placenta, is as little 

 natural as that which would be composed on the basis of the 

 diphyodont dentition, or the unguiculate feet. The association of 

 the Itodentia and Insectivora with the Quadrumana, as in the 

 latest modification of the placentary system f, is not likely to com- 

 mand acceptance. The diffused placenta, as in the Mare, Por- 

 poise, Peccari, Rhinoceros, and Camel, would lead to an equally 

 heterogeneous assemblage. In two well-defined minor groups, 

 e. g. the true Carnivora and the true Buminantia, there exist 

 characteristic modifications of the placenta, viz. the zonular and 

 cotyledonal respectively ; but though the zonular type is common 

 to the Carnivora, it is not peculiar to them ; it is that of the 

 placenta in the Hyrax and the Elephant, amongst the TJngulata, 

 So likewise the cotyledonal type characterizes the placenta of the 

 Sloth among the Bruta. 



Frimary Divisions of the Mammalia. — The question or problem 

 of the truly natural and equivalent primary groups of the class 

 Mammalia has occupied much of my consideration, and has ever 

 been present to my mind when gathering any new facts in the ana- 

 tomy of the Mammalia, during dissections of the rarer forms which 

 have died at the Zoological Gardens, or on other opportunities. 



The peculiar value of the leading modifications of the mammalian 

 brain, in regard to their association with concurrent modifications 

 in other important systems of organs, was illustrated in detail in 

 the Hunterian Course of Lectures on the Comparative Anatomy 

 of the Nervous System, delivered by me at the Eoyal College of 

 Surgeons in 1842. The ideas which were broached or suggested, 

 during the delivery of that course, I have tested by every subse- 

 quent acquisition of anatomical knowledge, and now feel myself 

 justified in submitting to the judgement of the Linnean Society, 

 with a view to publication, the following fourfold primary division 

 of the mammalian class, based upon the four leading modifications 

 of cerebral structure in that class. 



* Lectures on Comparative Anatomy, vol. iii. 4to. p. 445. 



t Q-EEVAIS, Zoologie et Paleontologie Fran9aise, 4to. 1853, p. 194. 



