gSo FRESH-WATER BIOLOGY 



20 (19) Shell thin, horny, smooth, or with fine radiating lines, which do not 



interrupt the margin Subgenus Liriola Dall. 



^=«jsi -<^, Two species are represented on the west coast. Example, Siphonario- 



^m--'-'-:^^^ peUoides Cpr., (Fig. 1416; x i5). 



Fig. 14 16. 



21 (18) Shell patelliform, obUquely conical. Animal with a lung only; no 



gill; no jaw Family Gadiniidae. 



Only one genus Gadinia Gray. 



Two species occur on the southern California coast. Another, de- 

 scribed from Cuba, may be loolied for on the Florida keys. Example, 

 G. reticulata Sby., (Fig. 14 17.) 

 Fig. 1417. 



22(3,17) Animal aquatic, inhabiting fresh water 23 



Four families: 23, 32, 50, 53. 



23 (32) Shell spiral, dextral; spire more or less elongated; tentacles flattened, 



triangular. Family Lymnaeidae. 



Only one genus Lymnaea Lamarck . . 24 



Eight subgenera: 24, 25, 26, 27, 28, 29, 30,31. 



24 (25) Shell large, thin; spire slender and acute; body-whorl large, inflated; 

 columella strongly twisted; axis pervious. 



'Tli Subgenus Lymnaea s.s. 



' ' '' - 



The typical species, L. stagnalis L., is circumboreal, but the typical form is 

 not found in America. The common American form is known as L. stagnalis 

 oppressa Say, (Fig. 141S; X7). 



Fig. 1418. 



as (26) Shell large, solid, bulimiform; spire short; body- whorl large, inflated; 

 axis impervious Subgenus Bulimnaea Haldeman. 



'\ \ M \r\ '^^^ typical and only species, Lymnaea megasoma Say, (Fig. 1419; X i), 



' , \\V \ Uy\\ inhabits the northern United States and Canada, west to Manitoba. 

 W^ o^^j^v'^ .' « Minnesota and Iowa. 



Fig. 1419. 



