118 FAMILY CHARACTERISTICS. 
as a test of the accuracy of the views I enter. 
tained 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. 
This bulging corresponds to the most prominent 
part of the gills, to which, in a large number of 
American Species of this Family, the eggs are ex- 
clusively 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 back- 
wards again, the outline meeting the side occu: 
pied by the hinge and. ligament, which, when 
very short, may determine a triangular form of — 
