202 The American Naturalist. [April,. 
of the elephants, and as its horizon is the oldest, I have sus- 
pected that it had well developed incisor teeth in the lower 
jaw, and have, therefore, placed it provisionally in the genus 
Tetrabelodon. It is probably ancestral to the M. ameri- 
cattus, but, perhaps, not through American forms, since none 
with the same type of molar have been yet found in the for- 
mations which intervene between those in which the two 
species occur. Such forms occur in Europe, as the Tetrabel- 
odon turicensis and the Mastodo?i borsoiii. Unless some spe- 
cies of synchronous age with these is found in North Amer- 
ica, we may suppose that the Mastodon americanus derived 
its immediate descent from Asiatic and European forms. 
With the Tetrabelodon angustidens Cuv. we commence the 
series in which the transverse crests of the fnolars have the 
appearance of being composed of distinct but appressed conic 
tubercles. In most of them, the valleys are more or less 
interrupted by tubercles. This is one of the most abundant,^ 
Original. 
as well as the most widely distributed species of the family,, 
extending its range from India to Central North America, 
through Europe. I have seen specimens from the Loup Fork 
beds of Kansas, Nebraska, and Dakota. Their size exceeds 
those of the typical European form, and the second (and 
probably third) true molars have a narrow fourth cross- 
crest. It is possible that it may become necessary, with more- 
complete information, to distinguish this form as a species^ 
