520 



MICHIGAN SURVEY, 1905. 



laterals, tricuspid, laterals quadrate, marginal teeth mde and low, multi- 

 cuspid. 



Shell minute, heliciform, depressed, umbilicated, aperture subcircular or 

 short oval, peristome elevated or straight, with or wdthout a whitish lip; 

 no apertural lamellae or folds. 



Genus VALLONIA Risso. 



Shell minute, openly and widely umbilicate, depressed; the spire low-con- 

 vex, consisting of 3-4^ whorls; color light and uniform; surface smooth or 

 ribbed; periphery rounded; last whorl usually descending in front. Aper- 

 ture oblique, circular or short oval; peristome continuous or nearly so, ex- 

 panded or reflexed, often thickened within. Foot small, short with no pedal 

 grooves; edges of sole somewhat crenulated; sole undivided; eye-peduncles 

 cylindrical,' not enlarged distally; tentacles short; labial 

 lobes well developed. 



Genitalia having the penis short with terminal retractor ; 

 epiphallus short bearing a flagellum. Dart sack present, 

 containing a straight, bladeless dart. No mucus glands. 

 Duct of spermatheca long, branchless. 



Jaw arcuate with a shght medina projection or none,, 

 sculptured with numerous (18 to 25) crowded, low riblets denticulating the 



margins. 



Radula having twenty-three to thirty^ 

 three teeth in a transverse row, median 

 teeth decidedly narrower than laterals, 

 tricuspid, the mesocone not half as long as 

 the basal plate, side cusps smaller. Later- 

 als with large square basal plates, the 

 Fig. 159. Dentition of Vaiionia. (Binney.) mesocone extending to its edge, ectocone 

 small; marginal teeth wide and low, multicuspid. 



Fig. 128. Jaw of 



V. pulchella. 



(Sterki.) 



KEY TO THE SPECIES OF VALLONIA. 



I. Surface smooth or very finely striate. 



a. Umbilicus regularly rounded pulchella. 



aa. Umbilicus elongate excentrica^ 



II. Surface with distinct, membranous ribs costata.. 



VALLONIA PULCHELLA (Muller). 



Shell moderately umbilicated, the umbilicus regularly 

 rounded; slightly convex above; straw-colored or whitish, 

 transparent, with fine and dense striae; whorls 3^-4, the 

 last comparatively large, well rounded, little expanded 

 toward the aperture; suture moderately deep; aperture 

 nearly circular; peristome abruptly everted, with a strong 

 white lip, thinner at either end. 



Alt. 1.2, greater diam. 2.4, lesser 2 mm. 



Fig. 160. V. pulchella. 

 (Sterki.) X 10. 



