832 QUADRULA 
it differs in being much smaller and frequently entirely devoid 
of plication on either its sides or posterior slope. A striking 
peculiarity is that the posterior end of the ligament is per- 
pendicularly over the centre of the base, whereas in the ordi- 
nary trapezoides the end is situated about three-fourths of the 
distance from the anterior to the posterior.” (Frierson). 
Type locality, Calcasieu River, La. 
Ouadrula trapezoides var. pentagonoides Frierson, Naut., 
XVI,. 1902, p. 40. 
Section QUADRULA Ss. Ss. 
Shell rhomboid, surface pustulous, with a high, rounded or 
sharp posterior ridge; epidermis generally smooth and bright. 
Type, Unio-cylindricus Say. 
Group of Ouadrula metanevra. 
Shell quadrate or rhomboid, with a wide, rounded posterior 
ridge, above which, on the posterior slope, is a decided radial 
furrow ; whole surface except the anterior end generally pus- 
tulous, the sculpture of the posterior slope being often wrin- 
kled; umbonal region high; epidermis shining, usually painted 
with a beautiful pattern of triangular spots, and sometimes 
chevron-shaped lines; hinge strong, the secondary lateral in 
right valve rather feeble, but there is often a faint third lateral 
above ; cavity of the beaks deep and compressed. 
Animal with the marsupium filling all four leaves of the 
branchie, the ova giving it a purplish tint ; inner gills free from 
the abdominal sac for the greater part of their length; mantle 
bordered with black; branchial opening: opposite the lobe of 
the shell, extending well on to its base; anal opening without 
papille. 
QUADRULA CYLINDRICA (Say). 
Shell much elongate, inflated and having parallel dorsal and 
ventral lines so that it is almost cylindrical, inequilateral, solid ; 
beaks rather full and elevated, turned forward over a deep. 
