VIVIPARA. 13 



hours every day by the fall of the tide. After the water has 

 receded they crawl for some time in the mud, and then burrow 

 into it, entirely concealing their shell, till the return of the tide. 

 The finest specimens are found on a sand bar in the middle of the 

 river, which is also bare at low water. They cease to hybernate 

 in the mud in very early spring, often within a few days after the 

 ice has broken up, and exclude the young shells of two or rather 

 more whirls, which are much oftener heterostrophe in the decisa 

 than when mature. The young have remained in the ovaries all 

 winter. The sexes are said to be distinct. 



The foot protrudes too far beyond the mouth to allow the 

 animal to eat while in motion. The figure is a fac-simile of the 

 outline of Haldeman's figure of the animal. The eyes will be 

 seen on an enlargement of the outer base of the tentacles. Fig. 

 49 represents a section of the lingual ribbon of V. decisa, for 

 which I am indebted to my friend Mr. E. S. Morse. The food is 

 supposed to be vegetable. Motions very slow. 



I have adopted the name Vivipara, which I find most generally 

 used. It was published without description or figure by Lamark 

 (Phil. Zool. 1809 II, 320), who substituted Paludina, giving a 

 description, in 1821. Meanwhile, Montfort had given a descrip- 

 tion and figure under the name Vivijoarus (1810. Conch. Syst. 

 II, 247, Ixii). 



The geographical distribution of the species is at present but 

 imperfectly known. In general terms, they may be divided into 

 the following groups : — 



1. Mexican. 



V. multicarinata. 



2. Universally distributed, ranging over the whole Union east 

 of the Rocky Mountains, and in Canada. 



V. decisa. 



3. Southern ; confined to the States bordering on the Gulf and 

 Southern Atlantic coast. 



V. cyclostomatiformis, V. wareana, 



V. intertexta, V. haleana, 



V. coosaensis, V. incrassata. 



