THE NATURAL HISTORY SURVEY. 295 
lunate, the terminations of about the same shape, but rather 
more expanded below; the mouth of the aperture dilates to a 
great extent, forming a bell-shaped projection, a little wider 
than high; peristome thin, sharp, somewhat expanded, the ter- 
minations joined by a distinct callus; just within the aperture 
there is a heavy ridge, where the bulging begins; the diameter. 
of the aperture, where the dilation begins, is but one-half that 
of the aperture at the ending of the dilation; interior of aper- 
ture bluish-white. 
Length, 6.00; width, 11.00; aperture length, 5.00; width, 4.00 mil]. (8481.) 
ue Geol ee 10.00; ce es SOO sr O00. “9 B481,) 
3 TAO ee 15.50; ‘ os DLO tm Oy nasi (B404,) 
ss OOS tess 14.50; by gs Aaa eta) Aon" | (BABS) 
Animal: Similar to that of wivolvis; color blackish all 
over; tentacles rather short and blunt; head very wide and 
auriculated; foot rounded before and rather pointed behind; 
) a = 
Fie. 99. 
Under surfaces of the animals of PLANORBIS, showing variation in the 
shape of the foot -~while in progression. (Original.) 1, BICARINATUS; 
2, TRIVOLVIS; 3, CAMPANULATUS; 4, TRUNCATUS., 
8 mill. long and 4 mill. wide. Heart beats seventy-two per 
minute. Some specimens appear of a greenish color when 
viewed through the shell. 
Faw: As usual. 
Radula formula: 43+3+4+4445434+19 (251-25); cen- 
tral tooth as usual; lateral teeth with three very nearly equal 
cusps; intermediate teeth and marginals as usual. The lateral 
teeth of this species differ considerably from those of “zvolvts, 
bicarinatus and truncatus in having three very nearly equal cusps. 
Several examinations have been made to verify this statement 
and there seems to be no doubt of the accuracy of the same 
(Fig. 98). 
Genitalia: Not examined. 
Geological distribution: Pleistocene; Loess. 
