4G2 



MICHIGAN SURVEY. 1905. 



II. Aperture bideiitate. 



a. Umbilicate sayana. 



aa. Imperforate elevata. 



III. Aperture unidentate. 

 a. Umbilicate. 



6. Single tooth on basal lip profunda. 



hi). Single tooth on parietal wall thyroides. 



aa. Imperforate. 



h. Body whorl wide, diameter twice the height alholahris dentata. 



hb. Body whorl ventricose, diameter less than twice the height, exoZeto. 



IV. Aperture edentate, 

 a. Umbilicate. 



b. Coarsely striate, lip broadly reflected thyroides. 



bb. Finely striate, shining, lip narrower; smaller dausa. 



aa. Imperforate. 



b. Diameter exceeding 20 mm., distinctly striate. 



c. Lip heavy, broadly reflected; uncolored albolabris. 



cc. Lip thin, narrowly reflected; usually banded midtilmeata. 



bb. Diameter not exceeding 17 mm. 



c. Globosely depressed, polished mitchelliana. 



cc. Convexly elevated, striate pen7isylvanica. 



' POLYGYRA TRIDENTATA (Say) 



Shell umbilicated, depressed; light horn or chestnut col- 

 ored spire very low; whorls 5^, rather convex, the last 

 scarcely deflected in front; aperture trilobate; peristome 

 white, flatly reflected, thickened within, with two acute 

 teeth on its inner margin, a strong oblicjue tooth on the 

 parietal wall. Alt. 8, greater diam. 16, lesser 14 mm. 



Fig. G. 

 P. tridentata. (Binney.) 



Helix tridentata, Say, Nich. Encycl., pt. II, fig. 1 (1817). 



Triodopsis , W. G. Binnev, Man. Am. Land Shells, 291, fig-. 312, 



(1885). 



Helix , Sager, Doc. H. Rep. :\Iich., 1839, 420, separate, ^14. 



, Miles, Rep. Geol. Surv. Mich., 236, (1861). 



Triodopsis , Currier, Shell-bearing Moll. Mich., 6. (1868). 



, DeCamp, Shell-bearing Moll. Mich., 7, (1881). 



Polygyra , Walker, Rev. Moll. Fauna, Mich., 17, (1894). 



Not a very abundant species. Saginaw-Grand valley and southeastern 

 counties north to Lapeer county. Not reported from the southwestern 

 part of the state, nor north of the Saginaw-Grand vallev 



POLYGYRA FRAUDULENTA (Pilsbiy). 



Shell umbilicated, depressed, globose, ribbed striate; reddish 

 horn colored; spire convex; whorls 6, rather convex, the last 

 deflected anteriorly, constricted behind the lip; aperture tri- 

 lobate, Avith a large oblique tooth on the parietal wall; peris- 

 tome reflected, thickened within, concave above, white, with 

 two teeth, the upper one decidedly inflected, the other sub- 

 basal, acute. Alt. 7^, greater diam. 13, lesser 11 mm. 



