Review of Recent Geological Literature. 403 
increase in size from the tiny Hyracotheriwm of the Wasatch Eocene to 
the great animals of Post-pliocene times. Mesohippus, however, has 
not reached a large stature, advancing beyond its Bridger predecessor, 
Pachynolophus, much less in regard to size than in morphological differ- 
entiation; the larger species of the Bridger genus are but little inferior in 
this respect to the smaller species of the White River form. In spite of 
its comparatively high degree of differentiation, Mesohippus was a very 
small animal compared with the recent horses, about the size of the New- 
foundland dog. The skeleton is essentially like that of existing Hquide 
in character and appearance, but presents many striking points of differ- 
ence.” Some of these points are as follows: Mesohippus as compared 
with Hqguwus is characterized by its smaller skull, shorter face, larger and 
more anteriorly and inferiorly placed, posteriorly open orbits, its less 
massive, less opisthocclous, cervical vertebrie, differently shaped odon- 
toid process, long and well arched back, less flattened ribs, its slenderer 
limbs and decidedly greater length of hind-limbs relative to fore-limbs 
than in Hguus, its complete though slender ulna, its functionally devel- 
oped second and fourth metapodials and splint-like metacarpal I. 
‘Concluding this part of his paper, the author observes: “There are 
thus many points of difference as regards the proportionate development 
of the various parts of the skeleton, between Meiohippus and Hquus, and 
these divergences, more especially the smaller and differently shaped 
head and the very slender tridactyl feet, give to the older type quite an- 
other physiognomy than that of the recent representatives of the group. 
even without taking into consideration its very much smaller size, 
Nevertheless, no one can examine the skeleton of the Miocene genus 
without being struck by its essentially equine nature; in the teeth alone 
is the fundamental similarity of plan not apparent at the first glance, 
though even here a careful examination reveals the connection very 
clearly. This similarity extends also to the earlier members of the equine 
series, for Hyracothertuwm from the lower Eocene belongs as unmistakably 
to this line as do any of the later genera, Indeed, one of the most strik- 
ing features of this phylum is the way in which its essential features, 
and even many apparently insignificant details, are, as it were, sketched 
out in very early times and then gradually elaborated, without deviation 
and without retrogression, until the final term of the series is reached.” 
In the second section of the contribution, nineteen pages are devoted 
to an account of the genus Leptomeryx, including description of nearly 
all of the skeleton with a restoration of L. evansi Jeidy, and a discus- 
sion of the problem of the systematic position of the genus. It is found 
that in twenty-seven characters that especially mark the tragulines, 
Leptomeryx agrees with them in twenty-one, presenting on the other hand 
six more or less important points of relationship to the Pecora. 
Boas’ view that the tragulines are a group of simplified ruminants de- 
rived from typical members of that series, by which Leptomeryx would be 
considered one of the direct ancestors of the tragulines, is rejected, and 
it is considered probable that “Leptomeryx is a side-branch of the trag- 
uline stem given off before the extreme concentration of the tarsus char- 
