42. Anterior part thickly pustulated; epidermis rayless; Mississippi drainage 



to Texas 



^uadrula verrucosa (Raf.) Pistol'grip, Buckhorn 

 Tritogonia verrucosa (Raf.) 

 Anterior part scarcely pustulate; epidermis with broken rays (color 

 markings) or green spots; central states 



^uadrula cylindrica (Say) Rabbit'foot 



43. Pustules or knobs grouped in more or less definite radial rows (If there 



is any indication of radial grouping, take this choice) 44. 



Pustules scattered over shell 51. 



44. Pustules mostly in one row down the center of each valve 45. 

 Pustules in two or more rows 49. 



45. Posterior slope with pustules which are usually arranged in rows; epi' 



dermis tawny to brown, with green spots; most of the Mississippi 

 drainage 



^uadrula metanevra (Raf.) Monkey-face 

 Posterior slope not pustulate 46. 



46. Interdentum very wide and flat; with a hump below each umbone; rays 



(color markings) spotted or broken and of varying widths; Tennessee 

 and Cumberland Rivers 



Dromus dromas (Lea) Dromedary Shell 

 Interdentum moderate to narrow; usually with a row of knobs down 

 each valve 47. 



47. With about three knobs on one valve distinctly alternating with those 



on the other; epidermis with fine broken or wavy rays; most of the 

 U. S. except the extreme eastern and western states 



Ohhquaria rejiexa Raf. Three'horned Warty-back 

 Usually with a row of knobs on each valve arranged almost opposite 

 each other or morcor-less irregularly; postbasal margin produced; 

 rayless or with faint rays (color markings) 48. 



48. Epidermis smooth, with faint rays; Ohio River drainage 



Dysnomia torulosa (Raf.) 

 Epidermis in very coarse, concentric folds, usually rayless; central states 

 Pleurohema cyphyum (Raf.) Bulbhead, Sheep-nose 

 Plethohasus cyphyus (Raf.) 

 (Plethohasus aesopus (Green)) 



49. With broken green rays; Ohio, Cumberland and Tennessee Rivers 



Cyprogenia stegaria (Raf.) Fan Shell 

 Cyprogenia irrorata (Lea) 

 Usually rayless; central states 50. 



50. Shell rounded; with several large pustules in two more or less distinct 



rows; postdorsal and dorso-posterior edges forming a right angle; 

 cardinal plate and laterals forming almost a right angle 



§^uadrula nodulata (Raf.) Warty-back 



^uadrula pustulata (Lea) 

 Shell angular; pustules many and small; angles, as above, greater than a 

 right angle 



§^uadru]a quadrula Raf. Maple-leaf 



153 



