THE NATURAL HISTORY SURVEY. 313 



tral fibrous projection from the superior surface; ends rounded 

 (Fig. in). 



Radula formula: JU+^+if-i-ir+V+- (190-1-190); 

 central tooth more or less quadrate, the lower outer corners 

 being very much attenuated; cusp nine-dentate, five denticles 

 being long and narrow and two on each side small and blunt; 

 laterals in two alternate series, the primary teeth large, ob- 

 liquely inclined, comb-like, five-dentate, with a varying num- 

 ber of small denticles between the five large cusps. The sec- 

 ondary teeth are long and narrow, with a wide, blunt cusp. 

 These latter, as also the central tooth, are very difficult to ob- 

 serve (Fig. in). 



Genitalia: Not observed. 



Distribution: Probably inhabits the whole of the Northern 

 and Central parts of the United States and Southern Canada. 



Geological distribution: Pleistocene; Loess. 



FIG. 111. 



Radula and jaw of PnvSA GYRINA Say. (Original.) c, central tooth 

 1, 2, 3, 4, various types of principal marginal teeth; a, secondary teeth; ; J; 

 jaw. 



Habitat: Found very abundantly in ponds and streams of 

 greater or lesser size, adhering to sticks or stones and crawl- 

 ing over the muddy bottom. 



Remarks: This is a very common and handsome species. 

 Its habits are active, moving with a rapid, steady, gliding mo- 

 tion. It is very interesting to watch a number of Physce in an 

 aquarium. As they are crawling along the bottom one will be 

 seen to suddenly rise to the top of the water and move along 

 with the foot applied to the surface, the shell hanging down. 

 Again, they may be seen descending, suspended by a thin 

 thread of mucus. When the animal rises suddenly, the bran- 

 chial cavity opens with a faint clicking sound, probably due to 

 the pressure of air in the lung. Physa frequently inhabits 



