PLANORBIS. 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. nautiteus (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. Ridges 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 striz 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. 
