Prof. Owen on the genus Mylodon. 101 
lidotherium (credits, femur, Onpiov, bellua,) manifests but a slight 
exaggeration of this character im its fossil thigh-bones. Nor can 
any of the known Megatherioids be termed other than great 
beasts, although the Megatherium proper best merits that deno- 
mination. In selecting, therefore, the term Mylodon for an ad- 
ditional genus to this extinct race, I had in view a principle of the 
nomenclature of the Megatherioid genera by which all the cha- 
racteristic peculiarities of the family are readily fixed in the 
memory. 
The second remark on which I beg to advert bears upon an 
anatomical point, but one which I believe to be of high iumport- 
ance. Dr. Wagner (p. 38), referrig to Prof. Mayer’s valuable 
remarks on the Anatomy of the Marsupialia, specifies those of 
the brain, im which, in opposition to Owen, he recognises conyo- 
lutions and a ‘corpus callosum,’ p. 38. I need only refer to my 
paper in the ‘Philos. Transactions,’ 1837, where the cerebral 
convolutions in the kangaroo and wombat are specially de- 
seribed, in order to demonstrate the want of a concomitant deve- 
lopment of the ‘ corpus callosum’ in those animals. 
The great transverse band or commissure which unites the two 
hemispheres, spanning from one to the other above the lateral 
ventricles,—which is plainly visible, as such, im the lowest Rodent 
or other Placental Mammal, with the smoothest, and, to outward 
appearance, sunplest brain,—this great commissure or corpus 
callosum, I again affirm, after reiterated dissections, to be absent 
mall the known genera of Marsupials. If the narrow transverse 
band, which unites together the hippocampi majores, at the front 
part of the fornix, be regarded, as I originally stated it might be, 
a rudiment of the ‘corpus callosum,’ the comparative anatomist 
is at liberty to apply that name to it. But, in pomt of fact, a 
great hiatus exists between the condition of the cerebral com- 
missures in the Implacental and that condition which we find in 
the lowest of the Placental Mammalia. The transitional struc- 
tures by which M. de Blainville traces a progressive deterioration 
of the commissural apparatus from Bats and Rodents to Marsu- 
pials I have not yet met with, and they seem to have equally 
escaped the observation of the able editors (Fréd. Cuvier and 
Laurillard) of the posthumous edition of Cuvier’s ‘Legons d’Anat. 
Comparée,’ 8vo, tom. 11. 184.5, who have subjoined the following 
note to the text of the great master :—‘“ Le corps calleux semble- 
rait étre en rapport de développement avec le corps strié, si Pon 
constatait dans d’autres animaux ce que montre déja le cerveau 
du dauphin, qui a des hémispheéres volumineux, un corps calleux 
proportionnellement petit et peu €pais, et un corps strié trés mé- 
diocre. Mais il y a, dans les Marsupiaux, une disposition re- 
marquable de l’appareil de commissure formé par la youte (fornix) 
