486 The American Naturalist. [June, 
. Some or all of the molars rooted. 
Last inferior premolar at least with four roots. 
Incisors with simple closed roots ; Atryptherude. 
> m 
AA. Inferior premolar with two roots. 
Incisors with simple closed roots, and no enamel ; 
Interathervide. 
Several incisors with open roots and an anterior enamel 
band as in the Glires; other incisors with closed roots; 
Protoxodontide. 
II. Molar teeth with simple open roots. 
A. Inferior molars curved outwards. 
Enamel covered by cement; Typotheriide. 
Enamel exposed ; Xotodontide. 
AA. Inferior molars curved inwards. 
Enamel not covered by cement; Toxodontiide. 
The skeleton is best known in the typical genera of the 
families Typotheriide and Toxodontide. In Typotherium 
there is a clavicle, and the femur has a third trochanter. 
The sacrum is elongate, including nine vertebrae, and the 
ischium articulates with the posterior of these, as the 
ilium does with the anterior. In Toxodon there is no 
clavicle, the femur has no third trochanter, and the ilium 
only articulates with the sacrum, which consists of five verte- 
bre. In both genera there is a central bone of the carpus. 
According to Ameghino, the families with rooted molars 
are of prior geologic age to those with the prismatic type 
with open roots. This succession is parallel to the history 
of the families of the Glires and the Diplarthra. The fol- 
lowing table of affinities and phylogeny is given by Ame 
ghino :— 
Typotheriidee Xotodontidee Toxodontide 
Protoxodontide 
Interatheriide Atrypotheriide 
