PHYSA. 95 



SHELL flat above, with a deep depression or concavity in 

 the middle, very concave below, rather solid for its size, and 

 opaque, yellowish-brown or horn-colour, closely and deeply 

 striate in the line of growth, but not otherwise sculptured : 

 epidermis rather thick : periphery rounded : whorls 8, ex- 

 tremely compact and much compressed, angular above and 

 slightly so beneath : suture deep : mouth semilunar, occasion- 

 ally strengthened inside by a rib : outer lip not reflected, the 

 upper side scarcely projecting beyond the lower one : inner 

 lip thin, not continuous but attached to the columella : 

 umbilicus large and deep. L. 0-075. B. 0-175. 



Var. albida. Shell nearly white. 



HABITAT : On water-plants in lakes, ponds, and 

 ditches throughout the greater part of Britain, and 

 reaching as far north as the Shetland Isles ; but it is 

 local. It is also an upper tertiary fossil. The variety 

 was found by me in a lake near Lerwick, with specimens 

 of the ordinary colour. The usual monstrosity occurs 

 in which the spire is dislocated. Abroad it ranges from 

 Siberia to Portugal and Corsica. 



This curious little mollusk is slow, irritable, and fond 

 of floating. It is not very prolific ; for each capsule (of 

 which it lays from 5 to 9 during the breeding-season) 

 contains only from 6 to 8 eggs, giving an annual average 

 of about 50 for an individual. 



The shell of this, as well as of the last species, is so 

 different from any other which I have described, that it 

 is unnecessary to make any comparison. Their forms 

 are, however, represented by many analogous species in 

 North America. 



Genus II. PHY'SA* Lamarck. PL IV. f. 4, 5, 6, 7. 



BODY rather long, and twisted in a spiral coil : tentacles 

 long and slender : foot rather long, rounded in front and 



* A bladder. 



