2o6 The American Naturalist, [April, 
the tuberculated crests of the T. serridens, and no important 
characters appear to separate it from the latter. 
The Mastodon mirificus Leidy is known from a left ramus 
of a lower jaw, which supports the last molar. The interme- 
diate molars are probably four-crested (tetralophodont), and 
the last molar has six crests, and is a large tooth, occupying 
the entire dentary portion of the lower jaw. In this respect 
it differs from the Tetrabelodons campester and longirostris, 
where the fifth and sixth molars are in simultaneous use. 
The crests are divided on the middle line, and each half is so 
expanded as to close the intervening valleys very early in 
wear. Its symphysis is short and acute. Its nearest ally is 
the M. atticus Wagner, from the Upper Miocene beds of 
Pentelicus, Greece. 
Mastodon americanus Cuv. is the best known and latest 
in time of the American elephants. It is one of the 
largest species, and, after T. brevidens, possesses the simplest 
molar dentition. The symphysis of the lower jaw is short 
and decurved. The skull is wider and less elevated than that 
of the mammoth, and the tusks are shorter and less recurved. 
It was very abundant during the Plistocene age throughout 
North America, from ocean to ocean, and as far south as 
Mexico ; but it has not been found in the latter country. Its 
remains are usually found in swamps, in company with recent 
species of Mammalia, and with Eqtms fraternus dind Bos lat- 
ifrons. The carbonaceous remains of its vegetable food have 
been found between its ribs, showing that, like the mammoth, 
it lived on the twigs and leaves of trees. 
It is at first sight curious that this, the simplest of the fam- 
ily of elephants in the characters of its molar teeth, appears 
latest in time on this continent. But it must be regarded as 
an immigrant from the Old World, where an appropriate 
genealogy may be traced. Its nearest ally, Mastodon 
borso)iii, existed just anterior to it, during the Middle and 
Upper Pliocene, and this species was preceded in turn in the 
Middle and Upper Miocene by the T. turicensis, which pos- 
esses the same simplicity of the molar teeth. In its mandib- 
ular tusks the latter possesses another primitive character, 
which was nearly lost by its North American descendant. 
