62 HELICID^. 



probably as to all others. Tliej all retire into their 

 shells on the api)roach of -winter, and during seasons of 

 drought ; every part of the body is then retracted witlun 

 the plane of the aperture, and over it is extended a 

 membranous epiphragm, hke that of oui- Helices. They 

 cannot, however, retract the body much beyond the 

 plane of the mouth, and the foot is never wholly drawn 

 into the aperture of the mantle and concealed by it as m 

 Helix; the posterior extremity of the locomotive disk 

 being always visible, on a level with the mantle or collar. 

 The epiphragm sometimes possesses considerable thick- 

 ness and consistence. 



Nearly all the acknowledged species of this genus 

 inliabiting the United States were early described by 

 Mr. Say, and were characterized with his usual care. 

 After the pubhcation of these, and until the period of 

 his death, there was an interval when conchology was 

 but Uttle cultivated, or at least, during which no works 

 appeared embracing this genus. Smce that time a new 

 generation of naturahsts has come forward, mostly in the 

 northern States, who are diligently engaged in investi- 

 gating the zoology of the country. To them the works 

 of Jlr. Say have been almost the sole guide ; but the 

 only depositories of authentic specimens of his species 

 bemg the museum of the Academy at Philadelphia, and 

 the private cabinets of liis friends and contemporaries m 

 the same city, such specimens have remained almost 

 unknown elsewhere. Observers seeking to identify spe- 

 cies coming into their hands with those of Mr. Say, have, 



