35. — Helicodiscus parallelus (Say) 

 S6.—Vitrea 



Umbilicus closed; shell with remote regular longitudinal impressed 

 lines. V. indentata (Say) Pils. 



Umbilicus open; no impressed lines. V. hammonis (Strom.) Pils. 



37. — Zonitoides 



Diameter less than 3 mm. Z. minusculus (Binn.) Pils. 



Diameter more than 3 mm. 



Whorls slightly flattened; dimensions 5 x 4,5 x 2.75 mm. 



Z. arhoreus (Say) Pils. 

 Whorls more rounded; shell more robust; dimensions 7.5 x 6 x 

 3.6 mm. Z. nitidus (Muell.) Pils. 



38. — Stroiilops 



Shell whitish. S. virgo (Pils.) 



Shell brown, larger. S. lahyrinthica (Say) Pils. 



39. — Vallonia 



About 3 mm. in diameter. 



Whorls with numerous (but somewhat distant) fine transverse 

 ribs. F. gracilicosta Eeinh. 



Whorls without ribs. V. pulchella Muell. 



About 2 mm. in diameter. 



Nucleus with fine revolving striae; lip thickened and reflexed. 



V. parvula Sterk. 



Nucleus without revolving striae; lip thin and merely expand- 

 ed. V. perspectiva Sterk. 



40. — Polygyra. 



Less than 12 mm. in diameter; whorls and aperture narrow. 



Epidermis covered with hairs; umbilicus closed; aperture very 

 narrow, with notch in thickened peristome. 



P. Icirsuta (Say) Pils. 

 Epidermis without hairs; umbilicus partly opened; no notch 

 in peristome. 



Diameter about 12 mm. P. fraierna (Say) Pils. 



Diameter less than 10 mm. P. monodon (Rack.) Pils. 



More than 15 mm. in diameter; aperture wider. 



Umbilicus wide. P. profunda (Say) Pils. 



Umbilicus partly open. 



Diameter about 18 mm. ; no teeth in aperture. 



P. clausa (Say) Pils. 

 Diameter about 22 mm.; parietal tooth usually present. 



P. thyroides (Say) Pils. 



Umbilicus closed. 



Shell depressed; parietal tooth long, lamellar. 



P. appressa (Say) Pils. 

 Shell more elevated; parietal tooth short or wanting. 



Spirally marked with reddish lines and bands, sometimes 



85 



