313 



The endeavour to reply to this question led me to seek every opportunity of dissecting 

 the brain of Marsupialia, some of the results of which were communicated to the Royal 

 Society in October 1836. Subsequently, in my paper "On the Classification of the 

 Marsupialia" in the Transactions of the Zoological Society, vol. ii. (1839) p. 330, I 

 remarked " that in their cerebral conformation the Marsupialia manifest a close corre- 

 spondence with the Ovipara in the rudimental state of the corpus callosum ; the differ- 

 ence which the most closely analogous placental species offer in this respect is broadly 

 marked." 



Amongst the other structures there shown to be both common and peculiar to the 

 Marsupialia, I adduced the number of the true molar teeth, as characterized by size 

 and shape, remarking that " in the dental system itself, the varieties of which have 

 been chiefly appealed to as sanctioning the dispartition of the Marsupial order, we find 

 an important peculiarity, by which the carnivorous, omnivorous, and strictly vegetable- 

 feeding genera alike agree with each other, and differ from the corresponding placental 

 Mammalia. In the ordinary Ferae, for example, in the Quadrumana and in the 

 Rodentia, as likewise in the Pachydermata and Ruminantia, the number of grinders 

 developed on each side of each jaw, which are not subject to vertical displacement and 

 succession, is never more than three, while in the corresponding groups of Marsupialia 

 it is always four "\ 



Since the date of this paper (1839) the associated group of Marsupialia has not been 

 sought to be dissevered. It received the valuable sanction of Mr. Waterhouse in his 

 ' Natural History of the Mammalia;' and the generalization as to the number of true 

 molars is given amongst the characters of the order in Part I. of that work, which was 

 issued in 1845. 



1 Trans. Zool. Soc. vol. ii. p. 332. I noted here the exception, previously pointed out by 3Ir. Waterhouse. 

 in Petaurus (Acrobates) pygmams, also that of the similarly minute Phalaarjista gUriformis, pp. 325-333. 



