EESEARCHES 



UPON THE 



HYDEOBIIN^ 



AND ALLIED FORMS. 



INTRODUCTORY REMARKS. 



Abounding in the fresh waters of North America, and in some 

 instances upon land contiguous to fresh water, there are to be 

 found species of minute shells resembling the Vivijjari in form, 

 but differing from them in the spiral form of the operculum. 

 Some of these shells have been erroneously referred to the genera 

 Paludina, Melania, Leptoxis, and Cyclosfoma. To others the 

 generic names AmnicoJa, Pomatiopsis, Somaiogyrus, and ChiJo- 

 cyclus have been applied. They must all, I believe, be referred 

 to the extensive family Rissoid^, a group of gasteropods re- 

 markable for their small size, which are very generally distributed 

 throughout the globe, and are very numerous in the sea, as well 

 as in fresh waters. 



The Rissoidfe were very properly separated as a family from 

 the Littorinidaj by H. & A. Adams,* but the characters given by 

 these naturalists are not entirely satisfactory. The character 

 " rostrum more or less adnate, below, to the fore-part of the foot," 

 is not a constant or general one, and some of the others given by 

 them are only partially distinctive, and can scarcely be relied 

 upon for the discrimination of families. The true distinction 

 between this group and the Littorinidte is found in the shape and 

 position of the teeth on the lingual ribbon. 



The Messrs. Adams include in their family Rissoidae the genera 

 Eissoina, Bissoa, Alvania, Onoba, Barleeia, Ceratia, Setia, Cin- 

 gula, Hydrohia, Skenea, and AmnicoJa. These are all probably 

 true Rissoids with the exception of Bai^leeia, which, belongs to 



Genera of Recent Mollusca, I, 327. 

 May, 1865. ( 1 ) 



