430 Bulletin American Museum of Natural History. [Vol. XXXIV, 
name a number of orders and suborders of extinct mammals, the Toxodontia, 
Typotheria, Astrapotheria, Entelonychia, Litopterna, all peculiar to South 
America. Amid endless variations in size proportions and specializations 
of one kind or another, all of these animals show in the fundamental molar 
pattern certain peculiarities not found in any of the many parallel adapta- 
tions among other mammals. There are certain characteristics in the 
lower dentition commion to all these South American Tertiary ungulates 
and unknown to any of the Holarctic orders. The most distinctive of these 
LEONTINIA 
PLEUROSTYLODON 
NotTostTyLops 
ARCTOSTYLOPS 
Fig.1. Arciostylops steini, type specimen, compared with South American Entelonychia. 
Outer views of lower teeth. 
is the cusp which rises in the middle of the talonid, branching off from its 
outer wall which forms a high curving crest. The construction is described 
by Schlosser! as follows: 
“Die unteren M bestehen aus je zwei ausseren Halbmonden von denen 
der vordere viel kiirzer ist als der hintere, und aus zwei mehr oder weniger 
komprimierten Innenhéckern gebildet wird, von welchen sich der vordere 
mit dem Hinterende des ersten Halbmondes innig verbindet, wihrend der 
1 Zittel’s Grundziige der Paleeont., Vertebrata, Ed. 1911, p. 512. 
