QUADRULA 839 
Margarita (Unio) stapes Lea, Syn., 1836, p. 15; 1838, p. 13. 
Margaron (Unio) stapes Lea, Syn., 1852, p. 22; 1870, p. 33. 
Quadrula stapes SIMPSON, Syn., 1900, p. 775. 
Unio retusus Say, Am. Conch., VI, 1834. 
Unio stegarius Kuster, Conch. Cab. Unio, 1861, p. 211, pl. 
Lexy Ho. 
In this species the beaks are placed usually midway between 
the anterior and posterior ends, sometimes a little in front 
of the center and occasionally just behind it. The shell is 
almost as decidedly truncate behind as is that of Plagiola ele- 
gans. ‘These two characters will distinguish it from all other 
related forms. 
Section THELIDERMA (Swainson, 1840) Simpson. 
Theliderma Swainson, Treat. Mal., 1840, p. 378.—SiMpsoN, 
Syn., 1900, p. 775. 
Shell rounded, quadrate to rhomboid, solid, inflated; beaks 
rather prominent; beak sculpture consisting of a few rather 
coarse, subparallel ridges; anterior end rounded or slightly 
subtruncate above; base often arcuate; posterior end truncate, 
high, angled behind the ligament; epidermis scarcely rayed, 
never painted ; beak cavities rather deep. 
Type, Unto lachrymosus Lea. 
Group of Quadrula lachrymosa. 
Shell quadrate or rhomboid, generally with a sharp, well- 
developed posterior ridge, in front of which 1s a wide, shallow, 
radial excavation; base incurved; posterior end sharply trun- 
cate, generally slightly biangular below, high and distinctly 
angled behind the ligament; posterior tubercles often in some- 
what radiating rows; earlier beak sculpture consisting of 
ridges nearly parallel with the growth lines; but later on be- 
coming doubly looped and blending into the general sculpture 
of the shell; epidermis smooth; pseudocardinals strong: lat- 
erals straight; nacre white. 
