ACADEMY OF SCIENCES. 95 



Some species appear to absorb the thickening of the lip when making a new 

 annual growth, while others retain it and thus show three or four dark bands, 

 corresponding to as many years' growth, and these multiplied bands form usually 

 a good specific character. Some probably exist for only one year, but this is 

 unsettled. 



Sculpture furnishes specific characters only in a few.. Other ^ood specific 

 characters appear to be those founded on the general form, the modifications of 

 parts, such as the columella, umbilicus, body-whorls, spire, number of whorls, 

 and proportions of length to breadth, mouth to length, or body-whorl to spire. 

 The authors who have adhered closely to these points have established the most 

 reliable species. 



Having studied specimens of all the following species and animals, except 

 those marked with a Q, I have arrived at the conclusions given in the synop- 

 sis as to their true position. As to most of the others, the accuracy of the 

 descriptions and figures given by their authors make their determination pretty 

 certain ; and if I have misunderstood them I do not feel accountable for the 

 fact, as imperfect descriptions are worse than none. 



Many of the higher divisions are founded in part on characters of the soft 

 parts, but are intelligible without these, which may be found described in most 

 recent works. 



As I propose, at some future time, to give the special distribution of our 

 species in a tabular form, with the numerous amendments required since the 

 publication of the Geog. Catal., I now limit myself to making out asynopsis 

 of the species and their leading characters, omitting much detailed criticism 

 of each, more suited for. a purely couchological journal. The present article 

 includes only the family Limne/dcc, which numbers nearly two-thirds of the 

 nominal species said to be found west of the Rocky Mountains, between lati- 

 tudes 32° and 49°. I have also collected materials for a similar synopsis of the 

 others, but not yet so complete as for this family. The plan followed is similar 

 to that used by me in Yol. Ill, p. 331, for the Hclicoids. 



The * indicates original measurements of authors, in hundredths of an inch. 



Order PULMONIFERA. 



Subord. LiMNOPHiLA. 



Fresh-water section, with shells. 



Fam. Limneida. Shell thin, corneous, elongated spiral, 

 planorboid, or patelliform. 

 Subfam. Limneince. Shell dextral, spiral, elongated, last whorl 

 usually very large, mouth oblong. 



Genus LIMNEA Earn. Spire attenuated, long, very slender, whorls 

 flattened, outer lip spreading, umbilicus scarcely perceptibly 



1 stagnalis Linn. Wh. 6, mouth about half total length; diam. 0.60 

 to 1, alt. 1.60-2.10. 



2 lepida Gld. Wh. 5, mouth a little over half of length, spire thicker in 

 proportion, otherwise like last ; diam *0.2.5, alt. *0.60. 



