THE NAIDES OF MISSOURI 183 
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 sulphur 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. Ortmann points out this genus as a con- 
necting link between the more primitive Unioninae and those of 
the type of the genus Pleurobema 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.’’) 
Ri XOG ih gen 0b 7 Any and, B. 
1834— Unto coopertanus Lea, Tr. Am. Phil. Soc., V., p. 61, pl. VIII, 
fig 27 
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 STRUCTURES:—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. 
