q8o fresh-water biology 



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



interrupt the margin Subgenus Liriola Dall. 



Two species are represented on the west coast. Example, Siphonaria 

 peltoides Cpr., (Fig. 1416; X li). 



Fig. 1416. 



21 (18) Shell patelliform, obliquely 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 looked for on the Florida keys. Example, 

 G. reticulata Sby., (Fig. 1417-) 

 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 Lytnnaca 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. 



\]] Subgenus Lymnaea s.s. 



V! it 



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. 1418; Xf). 



.-./ ji ' 



Fig. 1418. 



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



axis impervious Subgenus Bulimnaea Haldeman. 



I ' ^ \ \<K The typical and only species, Lymnaea megasoma Say, (Fig. 1419; X i), 



\ yy\\ inhabits the northern United States and Canada, west to Manitoba, 

 \\\^^^^^</ ' ^ Minnesota and Iowa. 



Fig. 1419. 



