314 



Monograph of the Families Rissoiidce, Jeffrey siidoe, Skeneidce, Cho- 

 ristidce and Homalogyridce. 



FAMILY RISSOIID^. 



Animal with a 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 (PL 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 Rissoiidse are small, often minute, phytophagous marine mol- 

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

 etc., from which they often suspend themselves by a mucous filar 

 ment. 



Formerly a large number of fresh and brackish-water forms were 

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

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

 apart Hydrobiidas: these will be considered in the series of the 

 Manual devoted to Fresh Water Mollusks. 



The Rissoiidae 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 Krister's Conch. Cab., 273 species, 1876. 



Si/nopsis of Genera and Subordinate Groups. 

 Genus RISSOIA, Fr&nenville. 1814. 



Shell im perforate 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 

 <3yes 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 



