94 



continuous, but closely attached to the columella and widely 

 spread on it : umbilicus broad and shallow. L. 0'35. B. 1. 



Var. albina. Shell perfectly white. 



HABITAT : Marshes, ponds, and ditches in many parts 

 of England and Ireland ; but although gregarious, it is 

 very local. It occurs in a fossil state in the mammalian 

 crag of Suffolk, as well as in the upper tertiary beds of 

 Suffolk, Essex, and Worcestershire. The variety is found 

 in Surrey. It is a Siberian species, and diffused over the 

 Continent as far south as Corsica. M. Terver has found 

 a thin variety of it in Algeria. 



This far exceeds in size any other European species of 

 Planorbis. Its anatomy, embryology, and habits were 

 accurately described by Lister nearly two centuries ago ; 

 and he seems to have made several experiments, but in 

 vain, with the hope of being able to fix and render useful 

 the purple dye which this mollusk so plentifully yields. 

 It is a sluggish and extremely sensitive animal; and 

 when irritated it emits the fluid or secretion in con- 

 siderable quantity from a gland at the sides of its neck. 

 It may often be seen floating on a warm and still summer 

 day. It lays only two or three capsules, each containing 

 from 20 to 40 eggs, which are excluded at the end of 

 fifteen or sixteen days. The epidermis of the young shell 

 is covered with fine down, its surface resembling velvet 

 pile. In this state it is the P. similis of Muller. 



11. P. CONTOR'TUS*, Linne*. 



Helix contorta, Lynn. Syst. Nat. ed. xii. p. 1244. P. contortus, 

 F. & H. iv. p. 160, pi. cxxvi. f. 3. 



BODY black, with a slight tinge of red : tentacles remark- 

 ably slender : eyes very small : foot broad and rounded in 

 front, narrowing behind to a blunt tail. 



* Twisted. 



