324 Io and its Habits. [June, 
suggests, it is perhaps appropriate in this connection to glance at 
the subject of geographical distribution as it relates to other 
forms, and suggest some of the conditions that seem to be corre- 
lated to the diverse forms. A little reflection will satisfy the 
most casual observer that the conditions under which Io is found 
are subject to variations of no small magnitude. First to be con- 
sidered is the climatic condition, affected by the combined influ- 
ences of latitude and elevation, conspiring in the northern limit of 
the region inhabited by Io to produce a lower mean annual tem- | 
perature than may be inferred at the southern limit. There may 
be in all a difference of four or five degrees of latitude, and possi- 
bly from two hundred to four hundred feet difference in eleva- 
tion. In the northern portion of the area inhabited by Io are 
found those forms destitute of spines, associated with others in 
which the spines are only rudimentary or reduced to mere tuber- 
cles. To the southward, the smooth forms diminish in numbers, 
and disappear entirely before the central latitude of Tennessee 
is reached, and as a warmer climate is approached the develop- 
ment of the spines becomes more and more luxuriant. This is 
true of the Holston River in Virginia and Tennessee, and recent 
observations have detected a similar state of facts in the Clinch 
River and its tributary, Powell’s River. Besides the influences 
of climate dependent on latitude and elevation, it may be con- 
jectured that there are other influences affecting Io, some of them 
in a considerable degree depending on the mineral properties of 
the water due to the variable qualities of rock and soil among the 
mountains of that region. In evidence of this may be suggested 
the differences in size and appearance of shells from different 
streams. 
. 
whole surface, occur in French Broad River. The most slender 
and graceful forms are found in Clinch River. The smallest adult 
forms occur in Powell’s River, where there are also other pecut- 
iarities observable that contrast curiously with what is know? of 
Io elsewhere. The tubercular and spinous forms of Powell’s River 
exhibit their characteristics (spines and tubercles) in a more mi 
. dimentary form than those found else where, and on averaging sir 4 
specimens it will be observed that they appear to have one se 3 
less, and more spines or tubercles on each whorl, than is observa of 
in the more luxuriant forms of other localities. How mu", 
all the differences in forms here suggested is ascribable to species: 
