2 RESEARCHES UrOX THE IIYDRODIIX.E 



a distinct family. But there are several genera to be added 

 to this list, which will be named further on in this paper. It 

 maj now be mentioned, however, that Lithoglyphus and Palu- 

 destrina, placed by the Messrs. Adams in the Littoriuidse, must 

 be included here, since they have the lingual dentition of the 

 Rissoidffi, the characteristic of which, as show* by Troschel, 

 consists in the presence of basal denticles on the rhachidian tooth. 

 "We also add Bythinella of Moquin-Tandon, usually considered 

 as synonymous with Hydrohia, or with Amnicola, but sufficiently 

 distinct. And the genus Pyrgula of Christofori & Jan, found 

 in the streams of Switzerland, probably also belongs here, although 

 hitherto referred by most authors to the Melaniida3.^ The same 

 may be said of Tricida, Benson, found in the rivers of India. 

 A. Adams has also added Fenella. The probabilities of the 

 genera Gecina and Blanfordia, A. Ad., and Gremnohates, Blan- 

 ford, also belonging here, will be discussed further on. 



TroscheP suggests the inclusion in the family of his groups 

 "Bythinioe, Lithoglyphi, and perhaps Truncatellce," which would 

 require the addition of the genera Bythinia, Stenothyra, Assi- 

 minea, TomicMa, and Truncatella. The two first named are 

 rightly, I think, here placed, in view of the structure of the 

 male organ and the lingual teeth ; but Assiminea differs con- 

 spicuously in the position of the eyes, and TomicMa and Trunca- 

 tella both in the position of the eyes and in the structure of the 

 breathing organs.^ Gray* includes in the family only three genera, 

 Bissoina, Bissoa, and Skenea ; — Lithoglyphus, Hydrohia, and 

 Amnicola being by him wrongly placed in the Littoriuida?. 



The following is offered as an amended character of the family 

 Rissoidos, which will serve to distinguish it from the groups allied 

 to it, or with which it has been confounded, as the Littorinidae, 

 Tiviparid?e, Truncatellida;, Melaniidffi, Valvatidse, etc. : — 



• Bourguiguat has already recognized the aflBnity of Pyrgnla to Hydrohia, 

 although he incorrectly places both these genera in the Viviparidse (Gue- 

 rin's "Revue et Magasin de Zoologie [2], XIII (1861), p. 526). 



2 Gehiss der Schnecken, I, 106. 



3 It may also be remarked that the dentition of the Truncatella group 

 is characterized by the presence of a transverse, continuous, dentated plate 

 beneath the cusp on the rhachidian tooth, which occurs in none of the 

 Rissoidffi.'* 



Guide to the Systematic Distr. of MolUisca, etc., I, 96. 



