Prof. Owen on the genus Mylodon. 101 



lidotherium {(TK€\h, femur, 67]pLov, hellua,) manifests but a slight 

 exaggeration of this character in its fossil thigh-bones. Nor can 

 any of the known Megatherioids be termed other than gi'eat 

 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 oq which I beg to advert bears upon an 

 anatomical point, but one which I believe to be of high import- 

 ance. Dr. Wagner (p. 38), referring to Prof. Mayer's valuable 

 remarks on the Anatomy of the MarsupiaUa, specifies those of 

 the brain, in which, in opposition to Owen, he recognises convo- 

 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- 

 scribed, 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, in the lowest Rodent 

 or other Placental Mammal, with the smoothest, and, to outward 

 appearance, simplest brain, — this great commissure or corpus 

 callosum, I again affirm, after reiterated dissections, to be absent 

 in all 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 point 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 (Fred. Cuvier and 

 Laurillard) of the posthumous edition of Cuvier's ^Le5ons d'Anat. 

 Comparee,' 8vo, tom. iii. 1845, who have subjoined the following 

 note to the text of the great master : — " Le corps calleux semble- 

 rait etre en rapport de developpement avec le corps strie, si Pon 

 constatait dans d'autres animaux ce que montre dejk le cerveau 

 du dauphin, qui a des hemispheres volumineux, un corps calleux 

 proportionnellement petit et peu epais, et un corps strie tres me- 

 diocre. Mais il y a, dans les Marsupiaux, une disposition re- 

 marquable de Pappareil de commissure forme par la voute (fornix) 



