THE MOLLUSC A OF GEORGIAN BAY 101 



SESSIONAL PAPER No. 39b 



the narrowly constricted throat just behind this. Aperture rhomboidal, narrowed 

 above. Narrowly rounded tops of the whorls all in the same plane. Lower surface 

 like trivolvis but narrow lower edge of the whorls more rounded and less angular. 

 Lines of growth coarse, regular and parallel. Adapted in the same manner as the 

 two preceding species to protected rather than exposed situations. Often distorted 

 so that the tops of the whorls are inclined at various angles. Varies considerably 

 in length of campanulate expansion and also in thickness of shell. Feeds on fil- 

 amentous green algie, diatoms and desmids. (PI. X, Fig. 1). 



Planorhis exacuous, Say.; occurs in protected weedy places, never in large 

 numbers. A well-marked species, having as distinguishing features a very sharp 

 peripheral carina, a lens-shaped shell and small size. Whorls flattened above, 

 broadly rounded below. Aperture triangular and very oblique. The greatly 

 flattened shell and very oblique aperture which allow it to lie close to the surface 

 over which it crawls would seem to adapt it to an exposed habitat, yet it was found 

 only in protected places. Varies in color, light coloured in sandy, and brown in 

 muddy places. (PL X, Fig. 3). 



Planorhis dilatatus, Gould, occurs on sticks along muddy river banks and in 

 muddy bays. Only a few obtained. Small brownish; top of shell flat; sharp 

 peripheral keel almost level with top of shell; whorls broadly rounded below; 

 aperture oblique; compared with P. exacuous it is smaller and higher in propor- 

 tion, the whorls are flatter above and much more convex below and the carina is 

 placed much higher. 



Planorhis hirsutus, Gould, occurs plentifully in weedy, sandy channels and in 

 muddy bays or in channels on smooth rocks covered with light deposit of sediment. 

 Easily recognized by the rough hairy shell. Shell wide and flat, having a sharp, 

 strongly deflected peripheral keel and a very oblique aperture. Surface covered 

 by crowded rows of hairs. Last portion of body whorl often strongly deflected. 

 Shell varies greatly with age. A shell of about three and a half whorls is concave 

 above and below, the aperture is only slightly oblique, the centrally-placed peri- 

 pheral keel is just appearing behind the aperture, the shell is high and all the whorls 

 are on the same level. In older shells the aperture becomes oblique, the shell be- 

 comes wide and flat, and there is a pronounced peripheral keel deflected down- 

 wards. The last whorls also drop below the level of the preceding whorls and the 

 shell becomes saucer-shaped. (PI. X, Fig. 2). 



Planorhis deflectus, Say, the most abundant Planorhis species of the region, 

 possesses a wider range of habitat than any other species, plentiful in quiet weedy 

 bays, in weedy sand channels and in shallow dark-colored pools on islands. Occurs 

 also on exposed shores. Recognized by its small size and rounded periphery. 

 Aperture only slightly oblique. Color varies from yellowish on lily-leaves in bays 

 to dark brown in dark island pools. Whorls in one plane or with last part of body- 

 whorl deflected downwards. Periphery sometimes flattened on its upper edge, 

 giving a peculiar sloping aspect to last whorl. Some specimens banded alternate 

 white and dark brown. 



