ACADEMY OF SCIENCES. 51 



is easy to see that the living shells, or their eggs, brought down by the great 

 rivers converging here, are likely to be caught by the rank growth of these 

 gigantic rushes, especially as the water must be much checked in rapidity of 

 flow by the meeting rivers, and by the sudden bend, at a right angle, toward the 

 west. Thus, they are compelled to remain, but probably many of them do not 

 increase in this region, as it is hundreds of miles distant from the points where 

 most of them were discovered, or have been before found. 



West of Antioch the marshes seem to become rapidly too brackish for their 

 existence ; and none live in those around Benicia— while Walnut Creek, empty- 

 ing nearly opposite there, is known to produce but ten species, of which three 

 are distinct from those of this list. 



It is possible that some of those obtained from a duck's stomach do not live 

 in the immediate neighborhood of Antioch. 



1. Succinea Oregonensis Lea, rare. 



2. S. Sillimani Bland. More common, (Rwl.) 



3. Arianta ramentosa GUI. Eight living on edge of marsh, large but thin, 

 approaching var. reticulata Pf. in form, (Ewl.) 



4. Aplodon Columbianus Lea, common, a thin, smooth variety. 



5. Limnea stagnalis Linn. Rare, but large and strong, living. 



6. L. lepida GUI. Rare, but exactly agrees, except rather darker in color 

 than types. A miniature of stagnalis, and may, perhaps, be only a dwarfed 

 race. 



7. L. humilis Say. Common, and nearly imperforate, probably the var. 

 called ferrnginea by Haldemann, (Rwl.) 



8. L. Nuttaliana Lea. Common, very large and fine, apparently quite dif- 

 ferent from elodes Say {palustris 31ull. ?). 



9. L. Tryoniana Lea. Rare and perhaps only a var. of Traskii Tryon, (Rwl.) 



10. L. desidiosa Saij. Common, and seem identical with Eastern types, but 

 usually more sculptured with revolving grooves, (Rwl.) 



11. Physa diaphana Tryon. Common, living in all the creeks running from 

 near Mt. Diablo. 



12. P. distinguenda Tryon. Rare, but large and perfect. 



13. P. Carltonii Lea, (Proc. Phil. Acad. Sc, 18G9). Described as new 

 from these specimens, and not found elsewhere. Approaches nearest to osculans 

 Hald., of Mexico, plicata Dekay, of New York, and nitens Phil, of Mexico, 

 {Bidinus nitens W. G. Biuney). Though very lustrous, the animal proves it to 

 be a Physa. 



14. Planorbis subcrenatus Cpr. .Common and of good size. 



15. P. lumens Cpr. Rare, but easily recognized. 



16. Helisonia amnion Gld. A few young specimens, not easily distinguish- 

 able from the last. 



17. H. tenuis? P/<//. (//. ?rn'o/'('/s from west" coast, of authors.) Referred 

 to tenuis by Tryon with doubt, and seems different. 



18. Gyraulus vermicularis Gld. Common. 



19. Cariuifex Newberryi Lea. A few very youug ones, perhaps a dwarfed 

 southern var., like those from Clear Lake. 



