PROSOTHERIUM 65 
and molar. A film of cement covers each tooth and extends 
to the top of the crown. The form is smaller than P. 
sculptum. 
MEASUREMENTS 
Upper premolar 3, length 6 mm., width 3; mm. 
Upper premolar 4, length 7 mm., width 33 mm. 
Upper molar 1, length 7 mm., width 33 mm. 
Prosotherium Ameghino 
Prosotherium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 426. 
In founding this genus, Ameghino says that lower pm. 
tis lacking, but our specimens show it present as a vestige, 
and also show no trace of lower inc. 3 against which Ame- 
ghino puts a question mark, making the formula > as 
given above. The upper molars are similar to those of 
Pachyrukhos except that they have an inner fold which 
has been lost in Pachyrukhos. Yhe premolars are unlike 
the molars. Lower molar 3 is three-lobed. The descrip- 
tion of the skeleton is given under the specific description 
of P. garzoni, and this shows a remarkable resemblance to 
the skeleton of Pachyrukhos, throughout, so that I have no 
doubt but that Prosotherium is the ancestor of Pachyrukhos, 
the changes in the teeth proceeding in the line of simplifi- 
cation which seems to be general in this order, and is in 
general characteristic of forms in which the teeth become 
rootless. 
Ameghino described four species, P. garzont, P. trian- 
gulidens, P. robustum, and P. quartum, the last two of 
which differ so little from P. triangulidens, that I can not 
consider them as independent species. 
Prosotherium garzoni Ameghino 
P. garzoni, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 426. 
This, the most abundant species of typotheres, occurs 
in our collection from the Chico del Chubut, west of Puerto 
5 
