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 mill. (8481.) 



6.50; " 10.00; " " 5.50; " 5.00 " (8481.) 



7.00; " 15.50; " " 5.50; " 5.00 " (8484.) 



7.00; " 14.50; " " 4.75; " 4.75 



Animal: Similar to that of trivolvis ; color blackish all 

 over; tentacles rather short and blunt; head very wide and 

 auriculated; foot rounded before and rather pointed behind; 



V 



* 3 



FIG. 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. 



Jaw: As usual. 



. Radula formula: i^+|+|+i+l+f+K(25- I -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 trivolvis, 

 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. 



