866 QUADRULA 
Animal having all four gills occupied throughout as a mar- 
supium, filled with pink or purplish ova when gravid; inner 
gills much the wider in front, free generally from the abdom- 
inal sac; outer gills cut away slopingly in front. 
Type, Unto trigonus Lea. 
Ortmann, (1. c.), raises this group to generic rank. 
Group of Quadrula beadleiana. 
Shell moderately solid, somewhat triangularly rounded, 
bluntly pointed and sometimes slightly triangular at post-base, 
with a more or less developed posterior ridge, in front of which 
it is full; beaks not very high; beak sculpture very coarse, con- 
centric ridges; disks irregularly, concentrically striate ; epider- 
mis rich, dark chestnut; hinge plate rather narrow; pseudo- 
cardinals radial, stumpy: cavity of the beaks only moderately 
deep. 
No gravid specimens have been seen, but those examined 
do not seem to differ from others of the genus. 
QuADRULA CHICKASAWHENSIS (Lea). 
Shell subelliptical to subrhomboid, compressed to subinflated, 
inequilateral, subsclid; beaks only moderately full or high; 
posterior ridge full, narrowly rounded or subangular, ending 
near the base in a blunt point: anterior end rounded; base 
curved; outline of dorsal slope curved, rarely subtruncate; 
surface with fine growth lines; epidermis dark brown, some- 
times tinted with green, often showing dark rest marks; pseu- 
docardinals triangular; laterals curved, sometimes partly dou- 
ble in the right valve ; beak cavities only moderately deep; mus- 
cle scars impressed ; nacre whitish, pink or salmon-tinted. 
Length 55, height 41, diam. 25 mm. 
Mississipp1 and Louisiana. 
Type locality, Chickasawha River, Miss. 
Umno chickasawhensts Lea, Pr. Ac. N. Sci. Phila., V, 1861, p. 
3907 Jl) Ae ONe Sc. Phila.= V5 1862, ps. 1Ol plain, ne eenore 
Obs WiRE 1862) p. TOs, plans noe sO: 
Margaron (Unio) chickasawhensis Lea, Syn., 1870, p. 55. 
OQuadrula chickasawhensis SIMPSON, Syn., T1900, p. 785. 
