GASTROPODA 541 



2. Lacuna Turton. Shell conical and thin with about 5 whorls, 

 with a long umbilical groove along the columella leading to a deep 

 umbilicus: 16 species. 



L. vincta (Montagu) (Fig. 839). Shell gray or yellowish in color, 

 with 2 dark bands on the upper whorl and 4 on the lower one, which 

 may be w^anting; length 12 mm.; diameter 7 mm.: circumpolar, south 

 to Long Island Sound and Puget Sound; very common; Europe. 



Family 4. EISSOIDAE. 



Shell small, spiral, turreted; aperture regularly rounded, with a 

 slightly-expanded lip; operculum horny; eyes at the base of the ten- 

 tacles; epipodium with filaments: marine or brackish water; over 200 

 species, principally towards the south. 



RiSSOA Freminville. Shell minute, conical, multispiral, white in 

 color: 75 species, mostly in the north temperate zone in 

 deep water. 



R. minuta (Totten). Shell thin and polished, with 5 

 whorls and an obtuse apex; color yellowish, which is 

 usually coated with a green pigment ; length 4 mm. ; diam- p. g^^ 



eter 2 mm. : New Jersey to Gulf of St. Lawrence ; common ^^Yv'^rriin ^"^ 

 on seaweed and under stones. 



R. aculeus (Gould) (Fig. 840). Shell thin, with 6 whorls, light yel- 

 lowish in color, with microscopic revolving lines ; length 4 mm. ; diameter 

 1.7 mm.: Long Island Sound to Greenland; common on seaweed. 



Family 5. HYDEOBIIDAE. 



Shell small, usually elongate and conical, with a pointed spire; 

 aperture rounded; operculum corneous or calcareous; umbilicus usually 

 open: about 80 species, in fresh and brackish water; cosmopolitan. 



Key to the genera of Hydrohiidae here described: 



©1 Shell ovate. 



6i Umbilicus closed. 



Cj In eastern and central states ." 1. Bythinia 



Ca On Pacific slope 3. Fluminicola 



6a Umbilicus open. 



Cy Spire well developed 2. Amnicola 



C2 Spire very low 4. Somatogyrus 



O2 Shell elongate. 



61 Umbilicus narrow and deep 5. Pomatiopsis 



hi Umbilicus absent or very shallow 0. Paludestrina 



1. Bythinia Gray. Shell conical, thin, covered with a thin perios- 

 tracum; operculum calcareous, not spiral, but with prominent growth- 

 ridges: in fresh water; 50 species, all in the Old World. 



