i 
324 Variation in Shells. [ ZOE 
male never possesses one, the female builds (constructs) one of the 
most delicate and beautiful found in the sea, for a nest, in which she 
deposits her eggs and hatches her young. This shell is not attached 
to the animal, and is an undoubted construction. From all the 
above quotations it will be seen we cannot call the shell a secretion, 
because secretions possess an inherent power of repair; their form | 
and size are determined by an inherent principle or law over which 
the animal has no control. We cannot call it an excretion, for ex- 
cretions are simply waste matter and without form. Construction, 
then, it seems to me, is the only term we can apply to it, because it 
is extravascular, possessing no inherent power of repair, no inherent 
power that determines its form or size, and is put together and 
moulded by the mechanical action of the mantle of the animal ; and 
a thing thus put together should be considered the object of the 
process that gives it its distinguishing character and name—shell. 
Our West Coast shells, with few exceptions, seem to present a 
_ greater amount of variation, or are in a more active state of tran- 
sition from one form to another, or perhaps our collectors are more 
lynx-eyed and push their investigations further im that direction 
than their co-laborers on the other side of the continent, for we hear 
but little about the variations of our Eastern shells, unless perchance 
some collector from this far-off but glorious land straggles into that 
field of research, while our West Coast shells are under considera- 
tion a greater part of the time. 
An exception, however, to some of the above remarks is found in | 
the Unionidae and the Strepomatide — two large families of fresh- 
water shells ‘‘ that form the special glory of North America,’’ on the 
east side of the Rocky Mountains, where the streams abound in 
species and varieties, and are fairly overcrowded with individuals, 
while the entire water system west of that great range and extending 
from Arizona to Alaska—an immense area —has rewarded the re- 
searches of the collector, so far, with a dozen or two forms of doubt- 
ful specific value, belonging to these great families. When we 
consider the great variety of forms and number of individuals of 
these classes of freshwater shells inhabiting the streams on the 
eastern side of the great divide, the paucity of species and individ-. 
uals of the same classes inhabiting the water-courses on the western ~ 
side becomes one of the most striking features of the study of Amer- 
ican conchology, and an explanation of its causes would be one of 
