MOLLUSC A OR UNIVALVES. 179 



octona, are found in England, among the roots 

 of trees at the base of limestone rocks. Many 

 of them, as the Achatina zebra, are covered 

 with a thick epidermis, but others are desti- 

 tute : a few are constantly reversed or sinistral 

 shells. The Achatina columnaris is one of the 

 most remarkable of land shells ; it is reversed, 

 and the columella forms a winding pillar, 

 visible within, quite to the summit of the 

 spire.* A small species found in the South 

 Sea Islands is strung by the natives, and used 

 for an ornament. 



46. Succinea. — Shell oval, small, thin 

 and transparent ; spire very 

 short, consisting of only two 

 or three whorls ; right lip 

 thin, left smooth ; aperture large ; no oper- 

 culum. Animal like the Helix. — There are 

 10 species, recent. 



The animal cannot entirely enter its shell ; 

 it lives upon herbs and shrubs on the borders 

 of rivulets, and appears to be almost amphi- 

 bious. It is found in England and other parts 



* Sowerby . 

 N 2 



