210 Remarks on Mr. Owen’s Letter, &c. 
Professor Owen proceeds to state, “'The Megalonyx laquea- 
tus of Dr. Harlan is a true Megalonyz,” [but which was not 
“mainly founded on the cast of a tooth.” | ‘Nor can I conceive 
any reasonable ground for its specific distinction from the Mega- 
lonyx Jeffersonii.” Perhaps not, if we confine our observations 
to teeth. Of the latter animal we have as yet only the bones of 
the fore-arm and hand, and a broken tooth, to rest our opinion 
on; I think, however, that in a comparison of the radius alone of 
the two specimens we might detect “ reasonable grounds for spe- 
‘cific distinction.” I have no means in reach of comparing the 
distal extremity of the tibia of the Orycterotherium Missourt- 
ense, with the same part in the Brazilian species which forms 
the type of Dr. Owen’s “ Mylodon.” _ : 
Dr. O. remarks at p. 345 of the letter above quoted: “ The 
tibia of the Missouri Mylodon corresponds with that of the Me- 
gatherium, in the deep ovoid depression at the anterior and inter- 
nal part of the lower articular end.” He also supposes that a 
similar structure will be found to characterize the tibia of the 
« Scelidotherium,” and farther on continues: “The Megatheroid 
family thus appears to have been as strikingly distinguished by 
this structure of the ankle-joint as the sloths are by the pivoted’ 
articulation of the astragalus with the fibula.” It is somewhat 
surprising that two experienced anatomists should differ so wide- 
ly on a structure submitted to ocular demonstration ; in the tibia 
of the Megalonys laqueatus originally described by me, as also 
in the specimen in possession of Mr. Graves, there is not the 
least approach to such a conformation. Nor will the tibia of the 
Megatherium show any such comparison with the same part in 
the Megalonyz ; they all differ widely in this respect from the 
Orycterotherium. 
Further on, p. 345, Dr. O. states: ‘In my report on the ‘ Mis- 
sourium,’ printed in the proceedings of the Geological Society, 
Dr. Harlan will find that I have duly acknowledged the origina- 
tor of the opinion that the ‘ T’etracaulodon’ was nothing but the 
young of the gigantic Mastodon. To Mr. Wm. Cooper of New 
York, the honor of this insight belongs.” ‘This paragraph in- 
volves two errors; the ‘ Tetracaulodon” was not founded on 
characters peculiar to a young animal ; nor was it first announced 
by Mr. Cooper that the ‘ Tetracaulodon” is only a variety of 
the Mastodon. That the four tusks exist in the skull of the 
