126 THE NAIADES OF MISSOURI 



ol 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 Lmnpsilis ("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, etc. 



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 aeration due to the action of papillae, car- 

 uncles, flaps, etc. 



1. EUipsaria-Groiip. Marsupium most primitive in that 

 the whole outer gill is occupied; yet advantage is secured for the 

 aeration 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 Cy progenia Aherii (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. 



