PLANOKBIS. 29 



so deeply sunk, in being more sharply keeled, and 

 in having a shallower umbilicus. 



It is a common species, inhabiting ponds and ditches 

 on weeds in most parts of the country. It is frequently 

 found covered with dirt, which is often extremely hard 

 to remove. 



3. P. NAUTILEUS (like a nautilus). 



Quoit- shaped, having the upper side flat and the under 

 side convex ; dirty white or brown ; striated in the line of 

 growth by ridges of epidermis ; whorls three. 



There is no mistaking this elegant little shell, which 

 is found on weeds and on the underside of decaying 

 leaves in ponds and ditches throughout the country. 



The variety, which is commonly found with the 

 type, is an exceedingly beautiful object under a lens. 



Var. cristata. Bidges of epidermis exaggerated 

 to points. 



4. P. ALBUS (white). 



Flattish above, with spire depressed; dull white, very 

 finely striated in the line of growth, but more distinctly 

 marked with raised striae spirally ; microscopically hispid ; 

 whorls five. 



This little shell often belies its name, being fre- 

 quently found black with dirt. Under a powerful 

 lens its delicate striations are visible, as well as rows 

 of minute hairs running in a spiral direction. 



