THE NATURAL HISTORY SURVEY. 301 



the lower carination; three on the peripheral wall, the two 

 upper ones being prominent, short, thick and triangular, and the 

 lower one more or less lamelliform and situated on the base 

 of the aperture, and one small conic tooth near the superior 

 junction of the peristome with the body-wall; peristome thin, 

 acute, slightly thickened inside, the superior margin a trifle 

 produced; interior of aperture pearly- white, with a band of 

 reddish just within the aperture extending parallel to the edge 

 of the aperture. In some specimens there is a sixth tooth, 

 small, acute, elevated, just below the large one on the parietal 

 wall; this, however, is not always developed (Fig. 101). 



Length, 2.50; width, 6.00; aperture length, 2.50; width, 1.25 mill. (8473.) 



" 2.00; " 5.00; " " 2.00; " 1.00 " (8478.) 



" 2.75; " 6.00; " " 2.25; " 1.50 " (8474.) 



" 3.00; " 8.00; " " 2.50; " 2.00 " (12363.) 



Animal: With a rounded foot which is rather wide; head 

 distinct, somewhat auriculate; tentacles very long and fili- 



FIG. 101. 



Aperture of SEGMENTINA ARMIGERA Say, showing number and posi- 

 tion of teeth. (Original.) 



form, one and one-half-times as long as the foot; eyes placed 

 as in Planorbis ; color blackish, lighter on edge of foot and ten- 

 tacles; respiratory groove long and pointed, thin and transpar- 

 ent. The foot measures 2 l / 2 by */ 2 mill. 



Jaw: Not differing from that of Planorbis. 



Radula formula: ^++1+1+1+^+ (18 I 18); central 

 tooth with a long, narrow base of attachment, expanded on the 

 lower, outer corners; reflection wide, bicuspid; lateral teeth 

 wide, a trifle longer than wide, tricuspid, the center cusp long, 

 wide and sharp, and the side cusps short and sharp; marginal 

 teeth at first similar to laterals with the addition of a second 

 small outer cusp; the marginals become wide and low toward 

 the margin and three small outer cusps are developed (Fig. 

 102). 



Genitalia: Not examined. 



