Molars with two cross-crests ; Megatherium C 
Molars simple ; last not elongate ; Scclidothcrium Ow 
Molars simple ; last elongate, grooved ; Mylodon Ow 
Rodimorpha. 
Molar teeth |. 
Pliomorphus Amegh. 
Lestodon Gerv. 
Molars with two cross-crests 
Molars simple, last elongate ; 
Molars simple, none elongate 
separated ; claws compressed ; Megalonyx Jeff. 
Molars simple ; canines close together ; Diodomiis Ameg. 
Besides these, there are several genera described by M. Ame- 
ghino, of w^hich the characters are not yet accessible to me. Thus 
Promegatherium'' differs from Megatherium by the presence of a 
band of enamel on the inner side of the molar teeth, but the num- 
ber of these is unknown. Promylodon^ Amegh. differs from My- 
lodon in the same way, but here also the number of these teeth, 
and other characters, are unknown. 
The genera of this family known to occur in North America 
are Megatherium, Mylodon, and Megalonyx. Of the first, a 
single species, M. mirahile Leidy, is known. It was a large 
animal, exceeding the Indian Rhinoceros in dimensions. Its re- 
mains have been found as yet only in the Pliocene of the South- 
ern States. Mylodon is represented by two species, M. laqiieatus 
Harlan, from the eastern region of the continent as far north as 
Pennsylvania, and M. sodalis Cope (Fig. i), from the Pliocene 
beds of Oregon. Both were large animals, not smaller than an 
ox in dimensions. Megalonyx is represented by a larger num- 
ber of species. The M. jeffersonii Cuvier has a geographical 
range nearly equal to that of the Mylodon laqtieahis, while another 
species (Plate XXXI.) has been only found in the Ticholeptus 
formation of Kansas. In Pennsylvania the bone caves have 
yielded the teeth of M. loxodon Cope, M. dissimilis Leidy, and 
M, tortidus Cope. 
