FAMILY LYMN^ID.^ 63 



Dybowski has recently applied the name Omplialolimnus to a 

 species of Lytnncea from the Crimea, which in outline resembles L. 

 stagnalis var. arenaria Colbeau, but which instead of having the 

 axis pervious and the pillar gyrate, as in most species of this type, has 

 the subumbilicate base and raised inner lip of the Radix section, to 

 which his L. lagorii probably belongs, although it has a more ele- 

 vated spire than most of the species of this section, being in this re- 

 spect intermediate between the latter and LymncEa proper. 



The existence of fresh water shells in lakes or ponds where the 

 water, tlirough evaporation, is gradually becoming more alkaline, has 

 been shown to be accompanied, in the lake-beds of the Great Basin of 

 the western United States, by a tendency to solidification, thickening 

 and corrugation or ribbing of the shells, regardless of their systematic 

 relations. This goes on until the alkalinity becomes so great that mol- 

 luscan life is no longer possible. We find in the fresh water Pliocene 

 beds of Utah, Lymna-a,, Pomp/io/yx, Carinrfex, Physa and Planorbis 

 exhibiting these changes as we ascend in the beds, until the latter be- 

 come barren of life. To these modifications we probably owe such 

 forms as Polyrhytis^ Plcurolimnaa^ V'orticifex, etc. I have shown 

 in another place' how such factors may be supposed to act in the case 

 of land shells exposed to alkaline dust on tropical islands such as the 

 Galapagos. While such changes are the result of the direct action of 

 the environment on the individual, and not hereditary or evolutionarj', 

 it is nevertheless convenient to recognize the results in the systematic 

 arrangement of the species. 



Disregarding synonyms, which can be deduced from the preceding 

 data, the general arrangement of the groups of the genus Lymncca 

 would be about as follows : 



Subgenus Lymnaea s. s. 



Section Lymnaa s. s. Shell thin, with an acute and slender spire 

 and expanded last whorl; the axis twisted, forming a (usually per- 

 vious) spiral coil without a true umbilicus ; the callus on the body 

 closely appressed ; the outer lip flaring more or less, simple, sharp, 

 normally without any varical thickening. T\i^^ L. stagnalis (Linn^). 

 Holarctic. ' 



Section Bulimnea Haldeman. Shell large and solid, bulimiform, 

 with an impervious axis, a twisted or subplicate pillar, the callus on 

 the body and pillar closely appressed, and the outer lip not thickened 

 or expanded. Type Lytnna:a fnegasoma Say. Nearctic. 



' Proc. Acad. Nat. Sciences Phila., for 1896, pp 406-426. 



