Dr. Harlan's New Fossil Mammalia. 345 



tinued from the upper and lateral parts of the base of the pha- 

 lanx, as in the Megatherium and Megalonyx, but were less 

 compressed, and presented the proportions of the claw-bone of 

 the Scelidotherium. (Fossil Mammalia of the Beagle, p. 97, 

 pi. 27.) The ungual phalanges of the Orycteropus have no 

 such osseous sheath. 



The tibia of the Missouri Mylodon corresponds with that of 

 the South American species of Mylodon, and with that of the 

 Megatherium, in the deep ovoid depression at the anterior and 

 internal part of the lower articular end, which therefore is not 

 peculiar to the Missouri Mylodon, although it forms a well 

 marked distinction between it and the Orycteropus. The as- 

 tragalus of the Scelidotherium, which has the same convex pro- 

 tuberance from the inner and fore part of its upper surface as 

 that of the Megatherium and Mylodon, must, therefore, have 

 governed the same excavation of the distal end of the tibia. The 

 Megatherioid family thus appears to have been as strikingly dis- 

 tinguished by this structure of the ankle-joint, as the Sloth's are 

 by the pivoted articulation of the astragalus with the fibula. 



The opportunity afforded me by Mr. Koch, of examining the 

 remains of the Mylodon Harlani, discovered by him in Benton 

 County, and which species I had before known only by Dr. 

 Harlan's figures, PI. XV, figs. 2, 3, 4, in the valuable work 

 already quoted, and by a drawing of the cast of the original spe- 

 cimen transmitted to me by Mr. Laurillard from Paris, fully con- 

 firmed the propriety of dissociating it from the Megalonyx laque- 

 attis or Pleurodon of Dr. Harlan, and established, by the dental 

 characters and those of the ankle-joint, the essential affinities of 

 the genus Mylodon to the Megatherian family. In my report on 

 the Missourium, printed in the Proceedings of the Geological So- 

 ciety, Dr. Harlan will find that I have duly acknowledged the 

 originator of the opinion, that ' the Tetracaulodon was nothing 

 but the young of the gigantic Mastodon.' To the excellent man 

 and naturalist, Mr. Win. Cooper of New York, the honor of this 

 insight belongs. I have had the same pleasure in acknowledg- 

 ing the ingenious observations on the mastodontal fossils in Mr. 

 Koch's collection, published in the Proceedings of the American 

 Philosophical Society, for October, 1841. 



I have the honor to remain, gentlemen, with much esteem, 

 your obedient servant, Richard Owen. 



Vol. xliv, No. 2.— Jan.-March, 1843. 44 



