314 RISSOIID^. 



Monograph of the Families Rlssoiidai, Jeffreysitdce, Skeneidce, Cho- 

 ristida'. and Hoinalogyridcc. 



Family RISSOIIDJE. 



Animal with ii more or less elongated muzzle ; elongated, cylin- 

 drical tentacles, with the eyes on swellings at their outer bases ; 

 verge (male organ) exserted, situated at a considerable distance be^ 

 hind the right tentacle ; gills both pallial, the right one largest^ 

 rather short and broad, composed of few laminae, which are much 

 broader than high ; foot oblong, punctate before, rounded or 

 pointed behind ; operculigerous lobe well developed. Operculum 

 corneous, paucispiral. Dentition (PI. 54, fig. 98.) 



Shell small, usually turbinate or elongate, often more or less um- 

 bilicated ; peristome continuous, more or less rounded, never truly 

 channeled in front. 



The Rissoiidie are small, often minute, phytoi)hagous marine mol- 

 lusks, inhabiting all parts of the world, frequenting algae, zostera, 

 ■etc., from which they often suspend themselves by a mucous fila- 

 ment. 



Formerly a large number of fresh and brackish-water forms wer& 

 here included, which, although nearly allied both as to animal and 

 shell, are now perhaps more properly considered to form a family 

 iipart — Hydrobiidaj: these will be considered in the series of the 

 Manual devoted to Fresh Water Mollusks. 



The Rissoiidie have been monographed by: Schwartz von Mohren- 

 stern, Denkschriften K. Akad. Wissen, 1860-64, 



Sowerby, in Reeve's Conchologia Iconica, xx, 123, sp. No date^ 



Weinkauff, in Kiister's Conch. C^ab., 273 species, 1876. 



Synopsis of Genera and Subordinate Groups. 

 Genus RISSOIA, Fremenville. 1814. 



Shell imperforate or subperforate, more or less oblong, turbinated, 

 usually thick, white or corneous, smooth, ribbed or cancellated, 

 aperture entire, oval, peristome thick, simple or reflected. Oper- 

 culum corneous, paucispiral (PI. 54, fig. 95). 



The animal has long, slender tentacles, sometimes ciliated, with 

 ■eyes on small prominences near their outer bases ; mantle with a 

 tentacular appendage in front, on the right side, or on either side ; 

 foot truncate in front, attenuated behind, with a glandular sulcus 



