MOLLUSCA OR UNIVALVES. 165 



porary epiphragma to protect them during 

 severe weather, which they throw off on the 

 return of warmth. The shells are spiral, with 

 no projecting parts on the exterior, excepting 

 those caused by increase of growth ; the right 

 lip or margin of the aperture is often recurved, 

 or reflected outwards. There are eleven ge- 

 nera. The animals of eight of these genera 

 have four tentacula, their eyes being placed at 

 the summit, the others only two. 



38. Helix. — Shell globular, convex, or 

 flattened; spire not much elevated ; mouth wide, 



not long, placed very obliquely to the axis of 

 the shell; lips disunited by the projection of 

 the last whorl, never pearly, but polished ; um- 

 M 3 



