246 The American Naturalist. (March, 
be an expression of those relations so far as they are under- 
stood and nota mere set of conveniences, based entirely upon 
the presence or absence, and similarity or dissimilarityfoi = 
parts. 
GEOLOGY OF THE REGION. 
In the immediate region in which the collections were made, 
only two distinct geological horizons appear on the surface, 
these are the Loup Fork and Equus beds. None of the water 
courses have here succeeded in entirely removing the Loup 
Fork, and exposing the underlying older strata. The Loup 
Fork beds consists of light colored, calcareous sandstones, 
somewhat loosely cemented, resembling in color and friability, 
old mortar. They are everywhere penetrated by numerous 
calcareous rods or tubes, probably the casts of root-stocks of 
aquatic plants. They dip very gently to the southeast which 
is evidenced by the fact that the southern slopes are gentle, 
while those looking northward are abrupt. Where they have 
not been entirely removed by erosion, the Equus beds one 
formably overlie the Loup Fork beds. This unconformity has 
been overlooked by all previous explorations in this region: 
Marsh makes no mention of it in reporting on his expedition 
into this very place in 1872; and in his subsequent descr 
tions of vertebrate fossils from these beds, he has not distin 
guished between them, although their respective faunas are 
really quite distinct, and the beds themselves are not the 
result of a continuous sedimentation from the commencement . 
of the one to the close of the other; but there was sa Ho 
ant break at the close of the Loup Fork when this ia 
became dry land, and remained such through a long period 
time, after which the Equus beds were deposited upo? 
eroded surface of the Loup Fork. a 
The Equus beds are composed of loose, incoherent er 
except for occasional layers of somewhat tough, gritty OT 
The rapidity with which they yield to erosion, and hed 
erally incoherent nature has greatly aided in concealing In 
exact statigraphic relations to the underlying bene 
