394 Charles Earle—Evolution of the American Tapir. 
more oblique than in the Wasatch Tapir, the last lower premolar is 
nearly transformed into the complexity of a molar. The last inferior 
true molar in this species is like that of the I. latidens in having a 
large third lobe. Until the structure of the upper premolars is 
known in the I. annectens, we cannot say with certainty whether 
this form led to the true Tapir. 
The structure of the manus in Isectolophus annectens is of great 
interest, and illustrates the small change which the foot of the Tapir 
has undergone since the Upper Hocene. This species has four well 
developed toes on the anterior foot, and the fifth toe is larger in 
comparison with the size of the third than in the recent Tapir. 
The upper and middle element of the carpus, the lunar bone, has 
two nearly equal distal articular faces; this character of the lunar 
showing that the four digits of the manus are more nearly equal in 
size than in modern types. 
In the evolution of the foot structure of the odd-toed Ungulates 
or Perissodactyles with the reduction of the toes, there has been 
a concomitant displacement of the carpal and tarsal elements. For 
example, one of the most primitive of the Ungulates, the genus 
Phenacodus, has these elements arranged one above the other, the 
serial order as it is called, and co-ordinated with this structure is 
the presence of five well-developed digits on each foot. In all the 
known species of the Tapir, however, there has taken place a high 
degree of displacement. We have observed! considerable differ- 
ence in the foot structure of two species of recent Tapir, but all the 
modern forms are more specialized in this respect than their Eocene 
ancestors. The most noticeable change in the relations of the bones 
of the carpus in the evolution of the Tapir series, is the change in 
extent of articulation between the lunar and magnum, and the lunar 
and unciform. In the more specialized forms the anterior articu- 
lation between the lunar and magnum is shut off entirely; these 
relations are connected with the reduced size of the fifth toe, and 
as a result the lunar comes to have a large articulation with the 
external and inferior. bone of the carpus, the unciform. These 
morphological characters are extremely important in tracing out the 
phylogeny of the Perissodactyla. 
There is a break in the ancestral history of the Tapir in 
America between the Upper Eocene and the middle layer of the 
White River Miocene; in Europe this record is more complete than 
in America. The earliest known species of Protapirus from America 
is the P. simplex from the middle layer, or Oreodon beds of the 
White River formation. The only known remains of this species 
are the superior premolars and fragments of the lower jaw. It is 
of surprising interest to find a Perissodactyle so high up as the 
White River beds, having premolars so simple in structure. In all 
the other Perissodactyla of this.epoch the premolars have become 
as complex as the true molars. In P. simplex the internal lobes of 
the superior premolars are absolutely simple in structure, and 
1 Charles Karle, Some points in the Comparative Osteology of the Tapir, Science, 
March, 1893. 
