PLANORBIS. 47 



broad, rounded in front, tapering gradually behind, and ending 

 in an obtuse tail. 



Shell flattish above with a depression in the middle, extremely- 

 concave beneath, thickish, brownish horn-colour, with fine, close- 

 set striae in the line of growth ; epidermis thickish ; periphery 

 rounded ; whorls 8, remarkably compact, considerably com- 

 pressed, angulated above, less so beneath ; suhire deep ; mouth 

 crescent-shaped, often furnished with a rib internally ; outer lip 

 very slightly produced, not reflected; i?iner lip thinly spread 

 over the base of the penultimate whorl, not continuous ; umbilicus 

 broad, deep. 



Inhabits lakes, ponds, and ditches, on aquatic plants 

 in most parts of Great Britain. It is, however, rather 

 local. It is a sluggish and irritable little creature, 

 shrinking from the slightest touch, and it frequently 

 floats in an inverted position on the under surface of 

 the water. The capsules of this species vary from five 

 to nine in number, each of them contains six to eight 

 eggs, and the fry are excluded in about ten or twelve 

 days. 



Var. albida. — " Shell nearly white." Found by Gwyn Jeffreys 

 in a lake near Lerwick. " Weston-super-Mare " (Rich). 



P. dilata'tus, Gould {P. lens, Lea). 



This American species was first noticed in this 

 country by Mr. Thomas Rogers, of Manchester, in 

 1869. There is every reason to believe that it came 

 across the Atlantic in cotton bales. Mr. Rogers 

 first found it in the Bolton Canal at Pendleton, close 

 to the refuse and warm water discharges from a cotton 

 mill, and afterwards in a similar situation in the canal 

 at Gorton. He sent specimens to Dr. Gwyn Jeffreys, 

 who, in a communication to the ' Annals and Magazine 



