490 The American Naturalist. [June, 
Incisors }; molars $; Pachyrhucus Amegh. 
II. Superior molars with three internal lobes. 
Incisors 2; molars 2; Trachytherus Amegh. 
Incisors 4; molars 3; Entelomorphus Amegh. 
Incisors }; molars 3; Mesotherium Serres. 
These genera are distributed as follows :— 
Hegetotherium: two species from the Lower Eocene. 
Pachyrhucus: five species Lower Eocene, four species Mio- 
cene, one species Inferior Pliocene. 
Trachytherus: one species Lower Eocene. 
Entelomorphus: One species Inferior Pliocene. 
Mesotherium: four species Upper Miocene, four species 
Lower Pliocene, three of them found also in the Upper Mio- 
cene. 
A clavicle is present in Pachyrhucus and Mesotherium, and 
may be expected to be discovered in the other genera of this- 
family. The dental canal sends out a branch which issues 
from the ramus posteriorly on the external side. According 
to Ameghino this character is not present in other families of 
the Toxodontia. i 
Most of the species were of small or medium size, and prob- 
ably resembled the conies in their appearance and habits. 
‘Mesotherium cristatum Serres, and Trachytherus spegazzianus 
Amegh. reached the size of the tapir. 
In the XoropoxntIDÆ of Ameghino we have a family which 
presents characters of both the families Mesotheriidæ and 
Toxodontidæ. As in the former, the inferior molars turn out- 
wards below, but they are not covered with cementum as in 
that family. The known genera have the dental series unin- 
terrupted, thus displaying a more primitive character than 
most of those of the two families mentioned. 
The genera are as follows :— 
I. Incisors entirely covered with enamel. Incisors and molars 
with open base; latter not plicate; Entomodus Amegh. 
II. Incisors with enamel bands only. 
« Molars z and ş trilobate internally and bilobate externally. 
All inferior premolars curved outwards; Xotodon Amegh. 
