72 NAIADES OF MISSOURI 



Opening by short mantle connection; gills long, wide anteriorly, 

 inner laminae of inner gills free from visceral mass; palpi short 

 and wide; only outer gills marsupial, when gravid the ovisacs 

 distend but little, giving the conglutinates a lanceolate shape; 

 conglutinates white, discharged whole; glochidium small, spineless, 

 subovate or slightly oblique; soft parts orange or sidphur color. 



Shell Characters: — Shell elongated to ovate; beaks 

 moderately high, sculptured by obscurely concentric ridges, 

 not extending out on disk; epidermis brown to yellow, usually 

 rayless; beak cavities moderately deep; hinge well formed; 

 nacre white to pinkish. 



As to shell characters this genus resembles both those of 

 Quadrula — particularly those of the pustulosa group — and also 

 the genus Rotundaria. Its marsupial characters show an advance 

 over the genus immediately preceeding. The bright coloration 

 of its nutritive soft parts and of its ova would suggest some affinity 

 with the Fusconaia. Dr. Ortm^nn points out this genus as a con- 

 necting link between the more primitive Unioninae and those of 

 the type of the genus Pleurohema and thus elevated Simpson's 

 section, Plethobasus, to generic rank, since Simpson recognized 

 special characters of the type, aesopus, in shallow beak cavities 

 and outer gills only as marupial. This genus is represented in this 

 state by aesopus, but doubtfully by cooperianus. 



Plethobasus cooperianus (Lea). 



("Cumberland Pig-toe," "Warty Pig-toe.") 

 PL XX, Figs. 57 A. and B. 



1834 — Unio cooperianus Lea, Tr. Am. Phil. Soc, V., p. 61, pi. VIII, 

 fig. 21. 



ANIMAL CHARACTERS. 



Nutritive Structures: — Branchial opening densely papil- 

 lose; anal and supra-anal separated by short (even deciduous) 

 connection; gills rather short, inner gills wider, its laminae free 

 from visceral mass; color of soft parts bright orange yellow, for 

 most part. 



Reproductive vStructures: — Marsupia only occupy outer 

 gills; when gravid swell moderately in center leaving ventral 

 edges sharpened; no glochidia found yet; its ova bright yellow, 

 giving the marsupia a sulphur color. 



