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 : ejpidei^mis 

 rather thick : periphery rounded : ivJiorls 8, extremely compact 

 and much compressed, angular above and slightly so beneath: 

 suture deep : mouth semilunar, occasionally strengthened in- 

 side by a rib : outer lip not reflected, the upper side scarcely 

 projecting beyond the lower one : inner lip thin, not con- 

 tinuous but attached to the columella : umbilicus large and 

 deep. L. 0-075. B. 0-175. 



Yar. alhida. 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 moUusk 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 

 diflPerent 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 pointed 



* A bladder. 



