64 LAND AND FRESH WATER MOLLUSKS 



Section Radix Montfort. Shell thin, usually with a short spire and 

 ample last whorl ; the axis twisted but not g)Tate, the outer lip often 

 expanded, the inner one more or less elevated and continuous across 

 the body, forming a more or less conspicuous umbilicus ; the outer lip 

 thin. Type Lytnncea auricularia (Linn^) . Holarctic. 



The umbilicus in this group varies from a mere chink to a rather 

 large orifice through which a bristle may be passed nearly to the apex 

 of the shell. 



Section Cyclolimncea Dall, nov. Shell thin, involute, the last whorl 

 as long as the shell, the outer lip thin, simple, not expanded, the inner 

 lip appressed, the axis not plicate, but with a small umbilical chink. 

 Type Lytnncea involuta Harvey. British. The mantle is said to be 

 extended partly over the shell. 



Section Polyrhytis Meek. Shell like Radix^ but axially strongly 

 ribbed. Type Z. /^/«^/ Meek. Pliocene, N. Am. 



Section Acella Haldeman. Shell thin, smooth, acute, extremely 

 slender; the aperture expanded at the margin, the inner lip not 

 appressed, a moderate chink behind it, the axis gyrate, pervious, 

 not plicate; the outer lip simple, sharp. Type L. gracilis Jay. 

 Nearctic. 



Section PleurolimncBa Meek. Shell like Acella^ but axially promi- 

 nently ribbed. Type L. tenuicostata Meek and Hayden. Eocene, 

 N. Am. 



Section Galba Schrank. Shell turrited, the whorls gradually in- 

 creasing, smooth ; the last whorl not inflated ; the aperture moderate ; 

 the outer lip not expanded or thickened ; the inner lip not appressed ; 

 the pillar not twisted or plicate, the axis minutely umbilicate. Type 

 L. truncatula (Miiller). Holarctic. 



Subgenus Stagnicola Leach. 



Section Stagnicola s. s. Shell elongate, smooth, the whorls gradu- 

 ally increasing, the last whorl moderate ; the outer lip sharp, not ex- 

 panded, with a varical thickening within, in the adult ; the pillar dis- 

 tinctly plicate, the inner lip appressed, the axis slightly or not at all 

 perforate. Type Z-. /a/e^^/r/V (Miiller). Holarctic. 



Section Leptolimnea Swainson. Shell like Stagnicola but more 

 cylindrical, with numerous whorls and a small aperture. Type L. 

 glaher (Muller). Palearctic. 



? Section Physastra Tapparone-Canefri. Shell like Stagnicola but 

 with a coarse dehiscent periostracum and coiled sinistrally. Type P. 

 vestita T.-C. Polynesian. 



