(32 HELICHLE. 



probably as to all others. They all retire into their 

 shells on the approach of winter, and during seasons of 

 drought ; every part of the body is then retracted within 

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

 membranous epiphragm, like that of our 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 in 

 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 

 inhabiting the United States were early described by 

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

 After the publication of these, and until the period of 

 his death, there was an interval when conchology was 

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

 appeared embracing this genus. Since that time a new 

 generation of naturalists 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 Mr. Say have been almost the sole guide ; but the 

 only depositories of authentic specimens of his species 

 being the museum of the Academy at Philadelphia, and 

 the private cabinets of his friends and contemporaries in 

 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, 



