e.

But it is sometimes exceedingly difficult to find the relation between
structure and form in Families, and I remember a case which I had taken
as a test of the accuracy of the views I entertained upon this subject,
and which perplexed and baffled me for years. It was that of our
fresh-water Mussels, the Family of Unios. There is a great variety of
outline among them,--some being oblong and very slender, others broad
with seemingly square outlines, others having a nearly triangular form,
while others again are almost circular; and I could not detect among
them all any feature of form that was connected with any essential
element of their structure. At last, however, I found this
test-character, and since that time I have had no doubt left in my mind
that form, determined by structure, is the true criterion of Families.
In the Unios it consists of the rounded outline of the anterior end of
the body reflected in a more or less open curve of the shell, bending
more abruptly along the lower side with an inflection followed by a
bulging, corresponding to the most prominent part of the gills, to which
alone, in a large number of American Species of this Family, the eggs
are transferred, giving to this part of the shell a prominence which it
has not in any of the European Species. At the posterior end of the body
this curve then bends upwards and backwards again, the outline meeting
the side occupied by the hinge and ligament, which, when very short, may
determine a triangular form of the whole shell, or, when equal to the
lower side and connected with a great height of the body, gives it a
quadrangular form, or, if the height is reduced, produces an elongated
form, or, finally, a rounded form, if the passage from one side to the
other is gradual. A comparison of the position of the internal organs of
different Species of Unios with the outlines of their shells will leave
no doubt that their form is determined by the structure of the animal.

A few other and more familiar examples may complete this discussion.
Among Climbing Birds, for instance, which are held together as a
more comprehensive group by the structure of their feet and by other
anatomical features, there are two Families so widely different in
their form that they may well serve as examples of this principle. The
Woodpeckers (_Picidae_) and the Parrots (_Psittacidae_), once considered
as two Genera only, have both been subdivided, in consequence of a more
int