814 QUADRULA 
Schluter applied the name Crenodonta to a group of Union- 
ide, the first species of which was the Unio plicatus of Say, 
but he gave no description of his group and did not designate 
a type. In 1853 Morch (Yoldi catalogue, p. 45) used this 
name without a description or a type, and it has been applied 
to the plicate uniones by von Martens (Biologia Centrali- 
Americana, Mollusca, 1900, p. 479). 
ORTMANN, (1. ¢.), raises this group to generic rank. 
Dr. Lea found in Quadrula heros the inner gills generally 
nearly or entirely free, but in certain specimens they were 
wholly united. My own experience in examining the animals 
of this species exactly coincides with his, thus showing that 
the character of the union of the inner gills with the abdom- 
inal sac, or their separation from it, is not a generic character,. 
as Agassiz beheved, nor is it even of specific value. 
Group of Ouadrula plicata. 
Shell rounded to subrhomboid; plications usually strong, 
oblique, though in occasional specimens the surface may be 
perfectly plain or slightly concentrically sculptured. 
QUADRULA PLICATA (Say). 
Shell subquadrate or subrhomboid, inflated, solid, inequilat- 
eral; beaks full and high, turned forward over a well-marked. 
lunule, their sculpture consisting of a few coarse, irregular, 
somewhat corrugated ridges; anterior end generally narrowed 
a little, rounded and often cut away somewhat below; base 
line rounded or nearly straight: dorsal slope often carried up 
into a low wing, obliquely truncated behind; posterior ridge 
rounded ; above it there is often a wide, shallow, radial depres- 
sion; surface with uneven, concentric sculpture and having 
usually several very strong, irregular ridges below the pos- 
terior ridge running nearly parallel with it; posterior slope 
with or without radial ridges; epidermis yellow-green, brown- 
ish or blackish, generally coarse and rough; left valve with 
two strong, radial pseudocardinals, which are often split into 
a number of radial denticles, with two strong laterals; right 
