lO NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA. 



iS.y^?)'^;/ /////; we however have not had the opportunity of com- 

 paring with authentic specimens of Prime's species. 



3. Vale of York — Newton Kyme ! 



4, //w/776e/-— Ferrybridge ! 



7. Went Vale — Stagnant pond, Ackworth, scarce, C.A. 

 10. Co//7e— Farnley Tyas, J.Wh. 



12. Lower Calder — Pond at Sandal. 



13. Airedale — Ponds, Halton I Canal, Kirkstall ! Stream, Roundhay ! Pond 



Scarcroft ! 



17. l-yens/et/t/o/e— Seven-ponds, North Stainley, J.D. 



22. Denuent- — Scarbro', W. B. 



23. Chalh Wolds — Spring-dyke, 'small, Avhitish and semi-transparent,' J.D. B. 



Var. Ryckholtii, Norm.=Sphasrium securis, Prime. 



The 6". securis of Prime, a native of North America, is con- 

 sidered synonymous with this variety by Dr. Jeffreys. 

 10. Colr\e — Farnley Tyas, J.Wh. 

 13. Airedale — Ponds, Halton ! 



■ :o: 



Genus PISIDIUM, C Pfeiffer. 



As in Sphmrium, there are several species of this puzzling 

 group common to both the Eastern and Western Hemispheres, 

 though hke Prof Forbes, Mr. Temple Prime, a great authority 

 on the Corbiculadee, regards the American forms as representative 

 only. 



The result of our own examination of the species which we 

 possess leads us to regard several of the forms as identical with 

 our species, a view in which we are supported by the opinion of 

 Dr. Jeffreys, Dr. Baudon, &c. 



Considerable confusion exists in this genus on account of the 

 great variability of the species, and consequently nearly every 

 author has different views of the limits of specific range. 



Pisidium amnicum, (Miill.) 



Pisidium dubium, Say. 

 According to Dr. Jeffreys, Pisiditun dubium of Say, perhaps 

 better known as P. virginicum, Bourg., is the same as this species. 



