Fresh-water Shells. 45 



folds of a bugle horn ; this shape would be termed 

 orbicular, from orbis — a sphere, or circular body. 

 This is the largest European species of Fresh-water 

 Shells so constructed; it is often found in deep 

 clear ditches, and yields a beautiful purple dye, 

 which, however, soon becomes dull, and changes; 

 it cannot be fixed, and is therefore valueless. The 

 mouth of this shell, in fine specimens, is tinged 

 with pale violet or lilac. 



There is another kind, the Keeled Planorbis 

 (P. lurinatus), which has the outer edge of the shell 



finely ridged, or keeled ; it is very small, and very 

 plentiful in fresh-water, both running and stag- 

 nant; where, too, is found the Common Physa 

 (P. fontinalis), the latter word meaning a spring 

 or fountain. This little mollusk is a quick and 

 active traveller, it sometimes comes out of its shell 

 and throws itself about in an extraordinary way, 

 keeping fast hold by its foot; the generic name, 

 Physa, would seem to have reference to the round, 



4 



