
1891.] The Litopterna. 689 
remarkable illustration of the identity of results produced by 
identical mechanical causes. The parallel is further increased 
by the diminution in the number of the digits, in the cases when 
the structure is known, in the course of geologic time. 
The families range from the Lower Eocene to Plistocene time. 
They have not been found hitherto outside of South America. 
They thus continued much longer than the Condylarthra, and 
must be regarded as a derivative of that suborder, which found 
in South America conditions favorable for continuance which 
were wanting in North America. 

_ FiG.1.—Proterotherium ? cavum Amegh. ; last three premolars, grinding face ; natural 
size. FIG. 2.—pitherium laternarium Amegh. ; true molars, from below; natural size. 
Both from Ameghino, s 
I now consider their characters in greater detail. 
In the PROTEROTHERIIDÆ we have a type more closely con- 
nected with the Periptychidæ of the Condylarthra than any later 
form. The superior molars are tritubercular, the protocone with 
well-developed accessory tubercles anteriorly and posteriorly, as 
in Periptychus. The external cusps of these teeth are modified 
into Vs, however, as in Meniscotherium. The three internal 
cusps become confluent on wear (Figs. 1, 2). 
The genera distinguishable in the material furnished by M. 
Ameghino are as follows : 
Digits? Symphysis not coössified; no in- 
termediate tubercles of the superior 
molars ; Proterotherium Amegh. 
Like the last, but intermediate tubercles 
present ; Anisolophus Burm. 
Symphysis mandibuli coössified ; Thoatherium Amegh. 
No intermediate tubercles ; ? symphysis ; 
three digits, the lateral rudimental ; Epitherium Amegh. 
