1897.] Toxodontia. 489 
III. Inferior molars rootless and with open base, and with op- 
posite lateral grooves. Anterior columns smaller than 
posterior ; shaft curved inwards; Gronotheriwm Amegh. 
Where the feet are known in this family, they exhibit three 
digits anteriorly and three posteriorly, which is remarkable in 
genera of such early age (Acrotherium, Adinotherium and 
Protoxodon). The possession of eight molars by Acrotherium 
is a remarkable fact, and one which reminds us of the Sirenia, 
to which Owen thought the Toxodontia to be allied. Ame- 
ghino regards them as representing five premolars and three 
true molars. 
The species of the above genera were of various dimensions, 
but generally exceeded those of the Interatheriide. The 
smallest is the Colpodon limitatum Amegh., which does not ex- 
ceed a rabbit, but other species equaled deer in dimensions, 
and the Protozodon sullivanii of Owen and Acrotherium rusticum 
Amegh. reached the size of the ox. 
In the increased development of the second incisor an ap- 
proach to the Toxodontide is seen, and the same tendency is 
exhibited in the open rooted and incurved crowns of the true 
molars in Gronotherium. Fifteen species of the family are 
known, of which four belong to Protoxodon and five to Adino- 
therium. 
Taking up another line of departure from the Interatheriidæ, 
we reach the TYPOTHERIID®, one of the two principal types by 
which the order Toxodontia was represented at the time of its 
extinction at the end of the Pliocene. We find here a great 
specialization of the first incisors in both jaws, and a gradual 
extinction of the third and second, so that in Typotherium 
this part of the dentition is decidedly gliriform, so much so 
that some authors have placed this family in the order Glires. 
In the oldest genus (Hegetotherium Amegh.) the molars are 
simple and of oval section, while in the latest forms they are 
lophodont with an external longitudinal and internal trans- 
verse crests, as in most other forms of the order. The genera 
differ as follows :— 
I. Superior molars without folds or columns. 
Incisors ;3; molars 4 Hegetotherium Amegh. 
