494 



MICHIGAN SURVEY, 1905. 



Var. catskillensis is a northern form, ranging the Grand Traverse region 

 through the upper peninsula, where it is the prevalent form. 



Subgenus PLANOQYRA Morse. 



'• Shell minute, orbicularly depressed, openly umbihcated, the spire flat, 

 whorls bearing thin, sharp, spaced laminae, parallel to growth stria?. , Aper- 

 ture rounded, lunar; Hp simple. 



PYRAMIDULA ASTERISCUS (Morse). 



Shell widely umbilicatecl, orbicularly de- 

 pressed, with from 25 to 30 delicate, thin, 

 transparent laminae parallel with growth lines, 

 with wavy edges and inclined backwards; 

 whorls 4, the upper one flattened, the last glob- 

 ose; suture deeply impressed; aperture sub- 

 circular; peristome simple, acute. 

 Fig. 84. p. nsteriscus. x26f. (Morst.) Alt. i; greater diam. 1^ mm. 

 Helix asteriscus, Morse, Proc. B. S. N. H., VL, 128, (1857). 



Patula , W. G. Binnev, Man. Am. Land Shells, 186, fig. 185, 



(1885). 



Pyramidula , Walker, Rev. Moll. Fauna Mich., 16, (1894). 



A northern species, which thus far has only been found at Charlevoix, 

 Isle Royale and the Porcupine Mountains, Ontonagon county. Yery mi- 

 nute, but very distinct in the position and character of the epidermal ribs, 

 which follow the lines of growth, differing in this respect from Zonitoides 

 exigua. 



Genus HELICODISCUS Morse. 



Shell small, disk or coin-shaped, with flat spire and 

 broad, shallow umbilicus; w^horls numerous, convex and 

 closely coiled, spirally striate or Urate, the last w^horl 

 having one or several pairs of tubercular teeth within, 

 situated upon the inner surface of the outer whorl; 

 aperture lunate; lip simple. 



The shell lies perfectly flat upon the posterior end of 

 Fig. 85. Animal of H. paraiieius. the f oot ; the eye-peduucles standing nearly vertical ; 

 '""^^' posterior end of the long and narrow foot conspicu- 



ously furrowed above, very short behind the mantle. 



Jaw arcuate, striate, the striae diverging some- 

 what from the median line; median projection 

 inconspicuous. 



Fig..86. Jim oi H. paraiieius. (Bmney.) Radula with ceutrals tricuspid, decidedly nar- 

 rower than the laterals; laterals with' large square basal-plates; marginals 

 low and wide, tricuspid. 



Fig. 87. Dentition of H. paraiieius. (Binney.) 



