102 THE AMERICAN MIDLAND NATURALIST 
are so inconstant that this subfamily may be termed a great 
group of intergrades. Having very typical hinge teeth and very 
closely adhering valves the branchial margins are not well papil- 
losed and the soft parts of the different species are more or less 
identical. In contrast with the other more modern subfamilies, 
Anodontinae and Lampsilinae, a greater differentiation of soft 
parts is noted in the latter, due to their more gaping valves 
and to a greater adjustment to aeration of the embryos; then, 
too, Unioninae differs from either of the two in that its breeding 
season is short (tachytictic), being confined to the summer. In 
the Unionae the color, form and solidity of the conglutinates can 
be considered as of greater systematic value than in the other 
sub-groups. It is to be noted that these summer breeders have 
the peculiar trait of aborting their conglutinates when they may 
be disturbed from their natural beds. The fact of the close, 
or even deciduous, mantle connection between the anal and the 
supra-anal openings may be a minor character in distinguishing 
the genera. The connection between inner laminae of the inner 
gills and the visceral mass may also serve in making distinction. 
From the fact that there are a great number of variations in shell 
character for this sub-family it is necessary to admit several 
genera so that there may not be so much opportunity for the same 
types of shell to turn up and thus give false impressions of rela- 
tionships. It is very striking to ngte the atavism of the spineless, 
subovate glochidium of this sub-family in the fact of its homolo- 
gous recurrence under the Lampsilis type. However, this natural 
reversion to primitive type in the embryo of the Lampsilinae is 
only an indication of the wide gap between the two sub-families 
‘as well as in the fact of its differences of physiological characters 
in the adult, such as the discharge of glochidia through the anal 
opening for the Unioninae and through the branchial for the 
Lampsilinae. However+the homologous differences in the soft 
parts and hard parts of the two groups are still greater than the 
analogous. Why more species of this primitive group should 
occur in the more modern region of this state (i. e., N. Mo., the 
New Prairies) than in that of the more ancient geologic formation 
(i. e., S. Mo.,.—the Ozark Uplift)—this is a problem that the 
author is trying to solve. The unusual variations within the 
sub-family especially is another problem that would also be 
solved, 
