MOLLUSCA OF MICHIGAN— WALKER. 



495 



HELICODISCUS PARALLELUS (Say). 



Shell widely umbilicatecl, discoidal; whorls 4, equally visible 



above and below, with numerous, equidistant, parallel, raised, 



^_^ revolving lines; suture much impressed; aperture remote from 



i^WjiJ axis, semi-lunate, narrow, not expanding; peristome acute, 



thin; within the aperture on the extreme circumference, are 



from one to three pairs of minute, conical, white teeth. 



p^^ gg Alt. H; greater diam. 3^, lesser 3 mm. 



H. parallelus. X5.7 

 (Binnev.) 



Planorbis parallelus, Say, Proc. A. N. S. P., IL, 164, (1821). 



Helicodiscus lineatus, W. G. Binney, Man. Am. Land Shells, 75, fig. 37, (1885). 



Helix lineata, Miles, Rep. Geol. Surv. Mich., 235, (1861). 



Helicodiscus , Currier, Shell-bearing Moll. Mich., 5, (1868). 



, DeCamp, Shell-bearing Moll. Mich., 6, (1881). 



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



Common and of general distribution. Easily distinguished by its plan- 

 orboid form and internal teeth. 



Sub-family PUNCTIN^E Morse. 



Jaw in numerous, distinct pieces, sometimes soldered together above, free 

 and imbricated below\ Marginal teeth quadrate. 



KEY TO GENERA OF PUNCTIN^. 



I. Shell minute, subdiscoidal, openly umbilicated 



II. Shell pupseform, perforate 



. . .Punctum. 

 Sphyradium. 



Genus PUNCTUH Morse. 



Shell minute, thin, subdiscoidal, but with convex spire, openly umbiHcated. 

 unicolored; whorls about 4, convex, the apical 1^ smooth, rather distinctly 

 demarked from the following whorls, which have oblique striae or irregular 

 riblets and excessively fine spiral striae; the last whorl cylindrical, not de- 

 scending in front. Aperture lunate, rounded; lip simple, thin. 



Jaw arcuate or horse-shoe shaped, com- 

 posed of numerous separate rhomboidal 

 plates bound together by a thin transparent 



1^ membrane, more or less overlapping, the 



outer imbricating over the inner plates ; the 

 Fig. 89. Jaw of P. pygmcexim. (Binney.) median two or three plates slightly sepa- 

 rated, not overlapping. 



Radula long and narrow; teeth rather separated, not in the least over- 

 lapping; centrals tricuspid, with narrow basal plate; laterals ^with wider 

 rhombic basal-plates and bicuspid; marginals not differentiated m any way 

 from the laterals, but becoming lower with shorter cusps. 



Fig. 90. D^r.titio-i of P. pygmceum (Binney.) 



