210 MEMOIRS OF THE CARNEGIE MUSEUM 
Among the fossils from the Titanotherium horizon in Montana are several speci- 
mens of Ictops. ‘The best of these are from the beds near McCarty’s Mountain, north 
of Dillon. Two species have been described by Dr. Matthew and myself from beds 
of the same age on Pipestone Creek in the vicinity of Whitehall. ‘The latter were 
described from fragments of jaws with teeth. One imperfect skull was found near 
Three Forks associated with Titanotheriwm. From the deposits near McCarty’s Moun- 
tain there are four specimens, all differing in size and in other particulars. All are 
represented by teeth, three by good parts of skulls, and two by skulls pretty nearly 
complete. ‘Two were from near the same level; one was slightly, and another con- 
siderably, lower. There appear to be similar characters in the specimens from the 
Titanotherium Beds which distinguish them from those of the Oreodon Beds, yet the 
latter have not been prepared and thoroughly studied. We have in the Carnegie 
Museum some good portions of skulls from the Oreodon Beds of Sioux County, 
Nebraska ; but they are either young, crushed, or injured in other ways, and are not 
in a condition for careful comparison. ‘The specimens now under consideration are 
not distorted, and most of the teeth are beautifully preserved. 
Though the specimens differ a good deal, they, perhaps, more resemble each 
other than they do the Leptictidee of the Middle White River. Perhaps the most 
interesting thing about these little animals, at the present, is a study of their varia- 
tions, and the light it throws, however little it may be, on the modes of change. 
It is evident that while the anatomical characters of some animals undergo 
local changes the other parts remain nearly the same; other animals are sub- 
ject to slower and more evenly distributed transformations. There are analogies 
between biological and geological changes. When a “zone of weakness” or a region 
of greater strain —or whatever it may prove to be—is developed, there is more or 
less instability afterward, and these are the regions that are subject to greatest change 
while other portions remain relatively stable. So when the tendency to vary has 
begun in some portion of an animal there is no knowing where it will end. Other 
animals do not seem to have any very plastic parts, but changes progress more 
slowly and uniformly. It is evident that neither conservatism nor extreme plas- 
ticity preserve races from extinction. 
So far as we know the Leptictidee appear to be rather conservative, though vary- 
ing considerably within certain limits. The present specimens differ so much, that 
it is more convenient to consider them as different species, which I think they 
really are. If all the intermediate varieties should be found, which is unlikely to 
happen, the species can be easily united. 
The beds in which the fossils oceur are more than 700 feet in thickness, and they 
