imiformly reddish or horn colored; diameter less than 

 25 mm. P. multilineata (Say) Pils. 



Without color bands; parietal tooth often present; diam- 

 eter more than 25 mm. (Our Iowa form=t'ar. alleni). 



P. albolabris (Say) Pils. 



41. — Succinea 



Spire very short ; body whorl large, shell ovate ; in swamps. 



S. retusa Lea. 



Spire more elevated; whorls more rounded. 



Less than 10 mm. long; spire elevated. S. avara Say. 



More than 10 mm. long. 



Whorls somewhat flattened, spirally grooved by irregular 

 lines. S. witteri Shim. 



Whorls rounded. 



Very oblique; more than 15 mm. long. S. ovalis Say. 

 More regularly conical; less than 15 mm. long. 



S. grosvenorii Lea. 



42. — Pisidium 



Length of shell about 7 mm. P. virginicum (Gmel.) Bourg. 



Length of shell less than 5 mm. 

 More than 3 mm. long. 



Beaks narrow, with wing-like appendages; shell compressed. 



P. compressum Prime. 

 Beaks rounded, not appendaged; shell more inflated. 



Beaks without ridges; shell dull. P. peraltum Sterk. 



Beaks ridged; shell polished. P. dbditum Hald. 



Less than 2 mm. long. P. punctatum Sterk. 



43. — Musculium 



Less than 5 mm. long. M. tenue (Prime) Sterk. 



More than 5 mm. long. 



Transversely elongated; beaks not central. 



M. transversum (Say) Sterk. 

 Rhombic or suborbicular ; beaks nearly central. 

 Length about 12 mm. 



Inflated; hinge margin not short. About 12.5 x 10.75 x 

 7.75 mm. M. partumeium (Say) Sterk. 



Less inflated; hinge short. About 12.5 x 10.75 x 4.5 mm. 



M. jayanum (Pr.) Sterk. 



Length about 9 mm. 



Inflated; about 9.25 x 7.75 x 6.25 mm. 



M. securis (Pr.) Sterk. 

 Not inflated; about 9.25 x 7.75 x 3.75 mm. 



M. truncatum (Lin.) Sterk. 



44. — Splmerium 



Shell rhombic ovate, highly polished, finely concentrically striate; 

 about 12.5 x 9.25 x 7.75 mm, S. rhomhoideum (Say) Pr. 



86 



