48 N'AIADES OF MISSOURI 



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. Pustnlosa Group. 



This group is mostly represented in this State by the northern 

 and western form, 0. 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), nodvilata (Raf.), fragosa (Conrad), aspera (Lea), 

 verrucosa (Raf.), nohilis (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 (vSay) and is characterized especi- 

 ally b}' 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. 



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 Amhlema 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. 



