314 THE AMERICAN MIDLAND NATURALIST 
j 
of tissue on the ventral border to permit a bulging out; when 
gravid ovisacs are undivided internally, and distal ends are extended 
beyond the original edge externally; glochidia of both Propterc 
(“‘ax-head’’) and Lampsilis (‘‘apron-form’’) types, varying much 
in shape an size; conglutinates white, undivided at their distal 
ends, discharged more or less broken through the thin ventral 
edges of the ovasacs; color of soft parts modest, never so bright 
with tinges of yellow or red as seen in the other sub-families, 
Unioninae and Anodontinae. 
SHELL CHARACTERS:—Shell rounded, sub-elliptical or elon- 
gated; beak sculpture generally obscure, when present usually 
the double looped type, rarely concentric; epidermis rarely dull, 
usually with bright color markings; hinge teeth rarely reduced, 
generally complete with well developed teeth; sex dimorphism 
of shell in most cases well expressed by a truncated or blunted 
posterior end, by an expanded post-ventral portion, ete. 
MISCELLANEOUS REMARKS:—With regard to marsupial 
structure the Missouri Lampsilinae naturally fall into three groups. 
All these agree, however, in the extension of the membranes 
beyond the ventral edge of the marsupium when gravid; hence 
this distention tends to make the membranes thinner so that 
osmosis may be facilitated. To aid further in this osmotic action, 
there is a tendency in the three following types to draw the mar- 
supium back toward the branchial opening where there is the 
greatest amount of aération due to the action of papillae, car- . 
uncles, flaps, etc. 
1. Elipsaria-Group. Marsupium most primitive in that 
the whole outer gill is occupied; yet advantage is secured for the 
aération of the embryos in rendering the ventral edges thin by 
distention and in throwing the marsupia into folds, thus increasing 
the surface for greater exposure to the water currents. The only 
representative in this state is E. clintonensis (Simpson). 
2. Obliquaria—Cyprogenia—Group. Number of ovisacs re- 
duced, but each greatly enlarged and elongated and placed at the 
greatest vantage point for oxygenation of the embryos. This 
group is represented in Missouri by only two species, Obliquaria 
reflexa (Raf.) and Cyprogenia Aberti (Conrad). The former has 
its few large ovisacs drawn back beyond the middle of the gill, 
while the latter has its ovisacs slightly in front of the middle of 
the gill, but extremely elongated into upward coiled spirals, 
