THE NATURAL HISTORY SURVEY. 293 



Animal: With a rather wide foot (6x35^ mill.) rounded be- 

 fore and behind; tentacles rather long and filiform, as long as, 

 or longer than, the foot; head rounded and not so much auric- 

 ulated as in trivolvis ; color blackish, flecked with white on the 

 foot and tentacles; respiratory tube large, blackish, flecked 

 with white; eyes placed as usual. 



Jaw: As in the genus. 



Radula formula: $j+f-ff+^++f+n (30- i -30); cen- 

 tral tooth as usual; lateral teeth of the usual shape, but reflec- 

 tion with a large, squarish central cusp, a large, rounded inner 



FIG. 97. 



Radula of PLANORBIS BICARINATUS Say. (Original.) c, central tooth; 

 1, first lateral; 9, intermediate tooth; 12, third marginal; 25, outer marginal. 



cusp and a smaller outer cusp; intermediate teeth with three 

 nearly equal, rather sharp cusps; marginal teeth as usual (Fig. 

 97). The writer counted 136 rows in one membrane. 



Genitalia: Not examined. 



Distribution: United States and Canada, from New Eng- 

 land to Manitoba, and south to New Mexico. 



Geological distribution : Pleistocene. 



Habitat: In rivers and ponds, in water from two to ten or 

 fifteen feet in depth, on a muddy bottom. 



Remarks: Bicarinatus is at once distinguished by the pecu- 

 liar, cone-shaped depression on both the upper and lower sur- 

 face, and by the characteristic v-shaped part of the aperture 

 which rises above the body whorl. The species is quite abun- 

 dant but is difficult to obtain alive on account of its preferring 

 deep water. The animal is rather slow in movement, the head 

 is carried much farther in advance of the foot than in trivolvis. 

 It is an interesting sight to see one of this species crawling up 



