GEOIvOGY OF LA COUNTY. 



107 



SUBGEUNS HEIvISOMA, SWAINSON. 



Planorbis trivolvis, Say.— Shell has four whirls, 

 which are finely striated, two and a half whirls visible 

 upon the rig^ht side, the ante-penultimate disappearing* 

 within the umbilical cavity, aperture larg^e, slightly 

 thickened within the marg-in, and its faces projecting- 

 beyond the planes of the shell; color chestnut: greatest 

 breadth three-fourths of an inch. Very abundant. 



Say gives the breadth as half an inch. Attains to 

 nearly an inch in breadth in Chicago River, and resem- 

 bles P. macrostomus. White. I have forms answering" 

 to Plan, corpulentiis. Say, which I believe to be tri- 

 volvis ; and also the following which are included in 

 the synonomy of Plan, trivolvis: Plan, reg-ularis, Lea 

 —an immature form; Plan, meg-astoma, DeKay; Plan, 

 trivolvis, var ; fallax, Halde. The varieties are 

 numerous. I believe that the differences may be 

 accounted for by the character of the waters, chemical 

 elements, station, climate, etc, 



Planorbis bicarinatus. Say. — Shell has three 

 whirls, carinated, color brownish; aperture larg-e; 

 color within reddish, with two white lines correspond- 

 ing- with the carina; breadth one-half inch. Abundant. 



SUBGENUS GYRAUEUS. AGASSIZ. 



Planorbis deflectus. Say. — Shell dextral, de- 

 pressed; whirls five, minutely wrinkled, texture thin, 

 aperture large, color light horn; breadth five-six- 

 teenths of an inch. Very abundant. 



Planorbis parvus, Say.— Shell horn color, whirls 

 five, minutely wrinkled; concave above and beneath; 

 and equally showing- the volutions; lip rounded; 

 breadth one-fifth of an inch. 



Ligamentina, Fleming. — Shell dextral, spire de- 

 pressed; whirls few, visible on both sides, furnished 

 internally with teeth; outer lip simple. 



