AND THEIR INHABITANTS. 213 



half of the preceding. It does not lay more than six 

 eggs. The shell is often infested with egg cases of a 

 water insect, or coated with confervas. 



P. nautilus (Linne) has the shell quoit-shaped; 

 the epidermis is rather thick. There are 3 whorls, 

 depressed above, the last exceeding in size the rest of 

 the shell ; the length is 0*035 ; the breadth, 0*1. 

 The variety cristatus has the transverse ridges stronger, 

 and the periphery notched by them. It is found on 

 aquatic plants in marshes from Zetland to the Channel 

 Isles. It lays from three to six eggs. Its small size and 

 dull appearance, with its large umbilicus, distinguish 

 it from the foregoing species. P. albus has a 

 dirty gray-coloured body, with black specks; the eyes 

 are small. The shell is flat, depressed in the centre ; 

 the epidermis is thick, and sometimes bristly. P. 

 glaber (Jeffreys) has a yellowish-gray body, rather short 

 tentacles, cylindrical, and ending in a blunt point; 

 the foot is rather broad; the size is 0*05 long, and 

 0*15 broad. It is also found on aquatic plants, but 

 it is not generally distributed. It is an upper tertiary 

 fossil. It is smaller than P. albus. P. spirorbis 

 (Mliller) has the body purplish-gray or reddish-brown, 

 with black specks on the foot. The shell is glossy, 

 brownish horn colour, and the epidermis is thin ; 

 there are 5 or 6 whorls, gradually increasing in size ; 

 the length is 0*04 ; the breadth is 0-25. The variety 

 carinatus has a smaller shell, with one whorl less than 

 usual. It is found on plants and grass from the 

 Moray Frith district to the Channel Isles. Like some 

 other species, in dry weather it retires within its shell, 

 and closes it with a yellowish lid until the rain comes. 



