200 The American Naturalist. [April, 
Edge of crest tuberculate T. serridens. 
aa. Crests transverse, composed of conic lobes. 
^. Valleys little uninterrupted. 
Last inferior molar narrow, with four crests ; an accessory tubercle in each valley; 
D. shepardi. 
/3. Valleys interrupted. 
Last inferior molar with four crests and a heel ; symphysis short, M. 150; smaller 
Last inferior molar with four crests and a cingulum ; symphysis longer, M. .280; 
size medium T. producius. 
Last inferior molar with five crests and a heel ; symphysis very long, M. .450 ; size 
largest T. angustidens. 
"ana. Crests broken into conic lobes ; those of opposite sides alternating. 
II. Intermediate molars with four transverse crests ; (tetralophodont). 
A long symphysis ; crests well separated, tubercular, with accessory lobes inter- 
rupting valleys T. campester. 
Symphysis very short; crests thick, closing valleys by contact; no accessory cusps; 
(Leidy) M. mirificus. 
III. Intermediate molars with 9-16 crests. 
/?. Valleys filled with cementum. 
Last molar with 18-27 cross-crests ; Elephas primigenius. 
The stratigraphic position of these species is as follows : 
Pleistocene. 
Elephas primigenius (less 1 
abundant). 
Pliocene. 
Elephas primigenius fmore 
abundant). 
Tetrabelodon serridens (hori 
zon probable). 
Dibelodon shepardi. 
Upper Miocene (Loup Fork). 
Tetrabelodon euhypodon. 
productus. 
angustidens. 
campester. 
Mastodon mirificus. 
Ticholeptus bed. 
Tetrabelodon brevidens. 
The horizons from which were obtained the Tetrabelodon ob- 
scuriis Leidy and the Dibelodon shepardi Leidy, are not suf- 
ficiently well-known. In the valley of Mexico, the D. shep- 
ardi \s from the Pliocene. No species of the order has. been 
found below the Ticholeptus beds ; a horizon about parallel 
