Charles Earle—Evolution of the American Tapir. 390 
Tn the latter group, the postero-external cone of the upper molars in 
later forms has a tendency to lengthen and become concave. The 
upper molars of Systemodon are provided with two complete 
transverse crests and there is a large antero-external cingular cusp. 
The lower series of teeth in Systemodon exhibit an interval or 
diastema behind the first premolar. This tooth is placed, however, 
close to the canine, which is considered to be a character typical of 
_the early Tapirs. The fourth inferior premolar in Systemodon is 
simpler in structure than a true molar; it has, however, two anterior 
cusps and a low heel. The latter is unprovided with a postero- 
internal cusp. 
In the further evolution of the Tapir this is the tooth which becomes 
molariform in structure. The last lower molar of Systemodon, like 
all the Hocene Tapirs, has a large third lobe. This character, and 
the less complexity of the premolars, sharply divides the Lower 
Eocene Tapirs in America from their Miocene descendants. Wortman 
and Harle! have lately shown it is probable that Systemodon had 
a rudiment of the fifth metatarsal on the hind foot. In the con-, 
temporary genus Hyracotherium of the Wasatch, this metapodial had 
disappeared. As far as vertebrate paleontology has progressed in 
regard to the fauna of the Wasatch Eocene, we may conclude that 
the genus Systemodon may be considered as standing in ancestral 
relations to both the Tapiride and Rhinocerotide. 
In the Wind River beds of America we meet with the genus 
Heptodon. This form has a superior molar indicating that it is off 
the line leading to the true Tapirs; the postero-external cusp has 
become concave, and reminds one of that occurring in the genus 
Hyrachyus. Heptodon is considered by Prof. Osborn to be the 
ancestor of Helaletes of the Middle Hocene. The Heptodon-Helaletes- 
Colodon line was a phylum of the Perissodactyla, contemporary 
with the Systemodon-Isectolophus- Protapirus line, but leading to a 
form (Colodon) more specialized than Protapirus. Colodon has 
lost the outer incisor of the lower jaw, and the fifth digit of the 
manus is much smaller than in Protapirus. 
There has as yet been no representative of the true Tapir line 
discovered in the Wind River beds of America. In the Bridger we 
find the genus Isectolophus, which should probably be placed in 
the line leading to the Tapir. The dentition of this genus is not 
yet fully known, especially the species from the Uinta. In Isecto- 
lophus latidens the third superior premolar has two internal cones, 
but the fourth tooth of this series is still simple in structure, like 
that of Systemodon. The true molars closely resemble those of the 
latter genus. The structure of the last inferior premolar of I. latidens 
is not known, but it was probably nearly molariform. The lower 
true molars show little advance over those of Systemodon; the last 
lower molar still retains the large third lobe. 
Referring to the Uinta Tapir, namely, Isectolophus annectens, we 
notice in this species that the external face of the upper molars is 
1 J. L. Wortman and Charles Earle, Ancestors of the Tapir from the Lower 
Miocene of Dakota, Bull. Amer. Mus, Nat. Hist. July, 1893. 
