304 JOURNAL OF CONCHOLOGY. 



PROBABLE CAUSES OF ABNORMAL VARIATION 

 IN LIMN^A 

 By B. STURGESS DODD. 

 Among our water snails, we are all aware, interesting abnormal 

 forms occasionally occur. In Limniza peregra and Limncsa 

 auricularia instances of strange more or less flattened expan- 

 sions of the outer lip are observed, and cases occur, when after 

 such expansions, the lip is even curled over in a most curious 

 manner. Jeff'reys has remarked, ' the consistency and even 

 the shape of the shells in this genus appears to depend much 

 on the nature and quantity of food, the chemical ingredients of 

 the water, and the degree of stagnation or rapidity of its 

 current' To account further for these remarkable expansions 

 it has been suggested that the pond, ditch, brook, or running 

 stream in which they are found, may at one time, owing to 

 deficiency of water, have left those animals in more or less con- 

 fined situations in damp muddy recesses, or on dry ground, 

 during periods of their most active growth, and that these 

 altered conditions would favour greatly an unusual expansion 

 of the animal's mantle while travelling in search of food at a 

 time when active shell secretion went on. 



' Litnnxa pe?-egra is known to be not very slow in its move- 

 ments (although sluggish at times), and nearly amphibious, ^s 

 its name imports/ says Jeffreys, ' and is fond of wandering, 

 being occasionally met with at some distance from its native 

 element, in a damp meadow, or climbing up the trunk of a 

 willow tree.' 



A temporary cessation of growth in Lwincea stagnalis pro- 

 duces a thickened lip, and in such cases varicose marks are 

 observable on the spire at intervals. 



An inner lip or rib is often formed within the shell of 

 Limncea glabra, while in Limncea palustris the whorls are some- 

 times distorted or scalariform. 



The views of others, with respect to the causes of variation 

 in the forms of the shell of our freshwater species, would be of 

 interest to the readers of your journal, 



J.C, iv., April, 1S85. 



