THE NATURAL HISTORY SURVEY. 307 



Man's Run (Shimek); Rock Island, 111. (Pilsbry); Rock Run, 

 Joliet, 111. (Ferriss). 



Geological distribution : Unknown. 



Habitat: Found at all seasons of the year adhering to 

 shells of Anodonta plana, sticks, leaves, etc. (Shimek.) Found 

 at Rock Run on sticks and leaves. 



Remarks: This comparatively rare species is known by its 

 very oblique shell; in some individuals the apex fairly over- 

 hangs the margin. It has been found only at Rock Run, Joliet, 

 but will probably be discovered at other localities by careful 

 and patient search. The shell has some resemblance to the 

 genus Gundlachia, and Mr. Pilsbry (Nautilus, IV, 48) says of it: 

 "Certain indications led me at the time of rinding this shell" 

 (speaking of a specimen found at Rock Island, 111.), "to refer 

 it to Gundlachia instead of Ancylus ; and it may be worth while 

 to follow this clew further. If my supposition proves to be 

 correct, Gundlachia will furnish the most extraordinary case of 

 dimorphism known among American mollusks."* 



FAMILY PHYSID^E. 



"Foot: Pointed behind. Buccal plate (when present) with- 

 out accessory laterals. Lateral teeth comb-like, alternating 

 with simple cusps. Base of central tooth broad, bifid; cusp 

 with several denticles." (Dali.) 



"Genifalia: On the left side." (Ball.) 



GENUS PHYSA Draparnaud, 1801. 



Shell: Thin, polished, sinistral; spire more or less acute; 

 aperture oval, rounded in front; colurnella provided with a cal- 

 lus which is spread over the last whorl; peristome acute. 



Animal: With a long foot, rounded before, pointed be- 

 hind; mantle reflected over a portion of the shell and fringed 

 with finger-like filaments; tentacles long, slender, pointed; 

 eyes situated at their inner bases; genitalia on leftside; jaw 

 chevron-shaped or absent, finely striated, with a central fibrous 

 projection; radula as described for the family (Fig. 107). 



The Physae are the most difficult to study of the fresh 

 water shells. So variable are their specific characters that no 

 less than seventy species have been described, and the sub- 

 ject is in such confusion that few students care to attempt their 

 study or collection. 



*See also The Nautilus, Vol. IX, p. 63, 1895, for further notes on this subject by Mr. 

 Pilsbry. 



