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 3j mm. 

 Upper premolar 4, length 7 mm., width 3 5 mm. 

 Upper molar I, length 7 mm., width 35 mm. 



Prosotherium Ameghino 



Prosotherium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 426. 



In founding this genus, Ameghino says that lower pm. 

 I is 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 3043 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. The 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. garzoni, 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 



