FAMILY PERISTOMATA. 197 



ings, foot divided in front into two crescent-shaped segments, 

 scooped out at the sides, branchiae plumose, sometimes exserted, 

 accompanied by a filiform tentacular appendage. Operculum 

 horny, concave, rather sunk in the aperture, concentrically 

 spiral, deeply ridged. 

 Shell ; globosely heliciform or discoid, of from three to five rounded 

 whorls of a semitransparent straw-colour; aperture small, with 

 the margin continuous. 

 Included in the family of Peristomata by Lamarck, Gould, Mo- 

 quin-Tandon, and others, is another freshwater mollusk, which 

 partakes only to a very moderate extent of the characters of 

 Pythinia and Paludina. Valvata has the same proboscis-like 

 head, and it is provided with an operculum, but in other respects 

 both the animal and its shell are distinguished by characters pecu- 

 liarly their own. The eyes of Valvata at the base of the tentacles 

 are on inner tubercular swellings, the foot is cleft in front into a 

 pair of crescent-shaped segments, the branchiae, most important 

 feature of all, appear in the form of an external plume, composed 

 of spirally twisted filaments, and accompanying this is a tentacular 

 thread, designed apparently both for its protection when exserted, 

 and for drawing the currents of water to it. The proboscis of 

 Valvata is almost as conspicuously developed as that of Cyclostoma. 

 In a beautifully executed drawing of V. piscinalis by Mr. Berkeley, 

 now before me, but in which the branchial plume is not sufficiently 

 exserted for illustration, the proboscis is rigidly protruded to half 

 the length of the tentacles. The eyes are placed more inwardly 

 than in Cyclostoma. 



Valvata, as may be seen on reference to our outline figures of 

 the shells, is small in Britain, and it is almost equally small in all 

 its places of habitation. V. piscinalis and cristata are very generally 

 diffused throughout Europe and Western Asia. Both species ap- 

 pear in Siberia, and as far southwards as Lycia and the islands of 

 the Mediterranean. Several other forms of Valvata, collected over 

 the same extended range, have been described as species, — Bai- 

 calensis, Gerstfeldt, Amoorensis, Bourguignat, prasina, Parreyss, 

 trochlea, Dunker, contorta, Menke, alpestris, Shuttleworth ; of none 

 of which have I been able to satisfy myself, yet it would be pre- 

 mature to include them in the list of synonyms. The North 

 American species are limited by Mr. Binney, in his ' Descriptive 

 Catalogue,' now passing through the press, to four : V. tricarinata, 

 sincera, and humeralis, Say, and pupoidea, Gould. The first of 



