130 THE AMERICAN MIDLAND NATURALIST 
disks usually sculptured; epidermis generally dark colored, ray- 
less or with greenish splotched paintings; cardinals heavy, double 
in both valves, ragged; laterals double in left, single in right; 
beak cavities deep, compressed or creviced; shells mostly not 
sexually dimorphic. 
MISCELLANEOUS REMARKS:—This genus naturally falls into 
three groups as follows:— 
I. Pustulosa Group. 
This group is mostly represented in this State by the northern 
and western form, Q. pustulosa schoocraftensis (lea), and is charas- 
terized by its greater inflation, smoother, larger and more elongated 
shell with beaks drawn back up more toward the center of the 
dorsal line; beak sculpture concentric. The actual typical pustu- 
losa is rarely, if ever, found in Missouri. 
II. ‘‘Lachrymosa” Group. 
This is represented in Missouri by Q. quadrula (Raf.) (=lach- 
rymosa lea), nodulata (Raf.), fragosa (Conrad), aspera (Lea), 
verrucosa (Raf.), nobilis (Conrad) and their intergrades, and may 
be characterized briefly by a somewhat quadrate or trapezoidal 
shell, profusely sculptured disk with tubercles arranged in two 
radiating rows from the beaks to ventral margin divided by a 
more or less broad radial furrow; beak sculpture double-loop 
type. 
III. Metanevra Group. 
This third group is only represented in this state by Q. 
metanevra (Raf.) and cylindrica (Say) and is characterized especi- 
ally by its height and coarsely sculptured umbonal ridge in front 
of which is a depression but no definite radial furrow and by its 
peculiarly triangular greenish splotches; beak sculpture double- 
looped or zigzag type. date 
The genus Quadrula tends toward an unusual intergra- 
dation of forms among the above groups in this state and because 
of this fact the genus might be more properly treated under various 
sub-genera for this catalogue; however, this treatment may be 
made unnecessary by the elimination of all the intergrades except 
those that possess the nearest approach to types. As to soft 
parts, this genus is identical with Amblema but is especially 
separated from the latter by the negative shell characters of 
oblique folds across the disk. Simpson, who bases much upon 
deep beak cavities, as one of the special characters of this genus, 
