138 PROCKEDINGS OF THE MALACOLOGICAL SOCIETY. 



buccal plate with accessory lateral jaws, tentacles flat and triangular, 

 foot quadrate ; habitat aquatic or amphibious. 

 There are tliree sub-families as follows: — 



Sub-family ACELLIXyE, n.sub-fam. 



Whorls appressed, coluniellar axis imperforate and twisted, produced 

 by a simple oblique reflection of the inner lip, aperture succiniform ; 

 luibitat deep waters of lakes. 



Genus, Acella.^ 



Sub-family LYMX.EIN^E, s.s. 



Whorls somewhat inflated, columellar axis sub-perforate and twisted, 

 with a more or less distinct marginal fold, aperture sub-auriculiform ; 

 habitat lakes and streams, generally in shallow waters. 



Genus, Lymnaa, s.s. 



Sub-family 



Whorls inflated, columellar axis straight and umbilicate, the 

 marginal fold obsolete, umbilicus partially hidden by a wide, smooth, 

 vertical expansion of the inner lip, aperture auriculiform ; habitat, 

 young stages passed in water, adults more or less amphibious in habits. 



Genus, Lipimcea; sub-genus, Galha. 



As witli other groups treated iu these pages, the old genus Lymntea 

 (fretjuently spelt Limnmi) has undergone various vicissitudes during 

 the last generation, and several classiflcations have been proposed, of 

 which that of Ball in 1905 is perhaps the best, though unnecessarily 

 elaborate for practical purposes. The North American species, which 

 number about a dozen, may be segregated into Acella, Haldeman, 

 a strictly American Province group, and a very ])rimitive one based 

 on a single species, Zyw««(«rf, s.s., which includes the larger species of 

 aquatic habits with a gyrate pillar, and Lymncea, sub-genus Galba, 

 which embraces the smaller amphibious species with a reflected pillar. 

 Other groups have recently been given generic rank, but space is too 

 valuable to devote to their discussion ; the new ones proposed will be 

 found in their proper places in the succeeding synonymy. 



Lynmcea, s.s., and Galha are circumboreal, and occur extensively in 

 the fossil state. Lymnoia ranges from the Mesozoic to the present, 

 wliile Galha is first known from the older Tertiary. Plearolimnea, 

 Meek, based on a Laramie and Eocene fossil, L. tenuicosta, M. & H., 

 from the Rocky Mountain region, has gone the rounds of the literature 

 unquestioned as a member of this family. Its striking resemblance to 

 Zaptychius, Walcott, of the Nevada Carboniferous, and Tortacella, 

 White, of the Utah Cretaceous, which together form a peculiar 

 group of Auriculoid pulmonates, apparently extinct, suggests that its 

 columellar characters should be carefully examined with a view of 

 redetermining its family position. 



^ Acella is included here merely to give an understanding of the classification. 

 No attempt is made to give a complete list of the exotic genera of the 

 various sub-families. 



