VOL. 1: | Variation in Shells. 327 
Anodonta angulata | found burrowing in beds of compact gravel 
and sand, in an upright or perpendicular position, with the small 
posterior end of the shell down and the broader or anterior end just 
level with or a little above or below the surface of the surrounding 
gravel beds. They seemed to prefer the steep sides of banks that 
led into deep water as burrowing places, rather than the flat bars, 
_although some were found in the latter places. I did not find them 
living in such large colonies as those in the San Joaquin River, yet 
their habits in that respect agreed fairly well. My previous impres- 
sion that they were somewhat sedentary was greatly strengthened 
by their burrowing habits and position in these gravel beds, and 
their somewhat gregarious habits here, especially when we remember 
that all sedentary mollusks are gregarious. 
With the knowledge of its habits and the factors in its environ- 
ment that act upon the shell, it is not difficult to recognize the causes 
that produce the remarkable angles or ridges which radiate from - 
the umbones to’ its anterior ventral margin, and at the same time 
mould the concave form of that part of the shell between its anterior 
margin and these ridges. 
It will be readily seen by their upright position in the gravel beds 
that the anterior or upper portion of the shell would be subjected 
to the continual friction of the current, sand, gravel and debris 
brought down the stream by the water, which in times of floods 
would be excessive. Freshets occur quite often in these mountain 
streams, especially in the spring or early part of the year—the season 
when mollusks construct the greater part of their shell—and conse- 
quently at this time these external conditions would have a greater 
moulding influence on the form of the shell than at any other time. . 
This rasping and wearing - away process, then, would require con- 
stant efforts on the part of the animal to rebuild and keep that part 
of shell in repair, and this constant direction of its energies to the 
upper or anterior end of the shell, if continued for a great length of 
time and through successive generations, would eventually result 
in a greater development of the organs of the anterior part of the 
body and a corresponding enlargement of the shell, probably at the — 
expense of the posterior or lower end, and more especially would - 
s this be the case when the shrinkage of the posterior end of the body 
and shell would be found to be an advantage in burrowing in the ~ 
compact gravel beds. _ ef 
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