IOWA ACADEMY OF SCIENCE 
179 
mains exposed for some time to atmospheric weathering and subse- 
cjnentlY transported by energetic gravel-bearing streams. 
Another point in favor of the Aftonian, rather than a pre-Aftonian 
age for tlie fossils is that some of the finds indicate that the bones had 
been fioated into place while yet some of the ligaments were intact 
and the skeleton was not completely dismembered. The teeth of Masto- 
don mirificiis, plate 27, were taken from a well, and in the limited space 
covered by the bottom of the well the workmen found the two upper 
molars, parts of the tusks, a large amount of cranial bones and other 
bones of the skeleton, that could not, by any conceivable probability 
have been there if the specimen had gone through the vicissitudes ex- 
perienced by the pre-Aftonian fossils. A more convincing case is that 
of the complete tusk of the American mastodon, unbroken, unmarred, 
eight feet in length around the curve and eight inches in diameter at 
the larger end, taken from Aftonian gravels penetrated in digging a 
well near Mapleton. With the tusk were found one upper tooth, a 
large amount of the cranium, bones of the legs and others, showing 
that a considerable portion of the skeleton has been deposited while yet 
the bones were held together. The pieces of the craninial bones show 
the socket for the tusk and sockets for the fangs of the teeth. The 
proximal end of the ulna is among the material taken from the well. 
The bottom of the well could cover only a small part of the space 
where the skeleton lay. ' Most of the breakage was evidently due to 
the well diggers. The Gladwin horse, plate 17, affords another illustra- 
tion of the same kind. A full set of upper and lower molars were 
found, lying in their natural relations to each other; with quite a large 
number of bones properly related, but too soft to be preserved except for 
some fragments of the lower ja^v. The chances that the true relations 
of so many parts of the skeleton could have been maintained through 
all the movements and processes to which they would have been sub- 
jected before reaching the position in which they were found, in the 
case of a horse that lived and died in preglacial time, are so few as to 
be negligible. 
The phalanx of Mylodon, plate 26, had the claw sheath almost com- 
plete when found. During the life of the animal this sheath was 
exceedingly vascular to afford nourishment to the horny nail or claw 
proper. After death the nail quickly decayed and left the sheath un- 
supported when, owing to the great number of vascular channels, it 
became exceedingly fragile. The specimen can hardly be handled 
without breaking off some of the sheath. Had this animal lived in 
preglacial time and been preserved in preglacial deposits, no part of 
