184 BRITISH MOLLUSKS. 



Sab. Banks of the Thames, hetween Greenwich and Woolwich. (In great 

 profusion, under stones and on mud, about the roots of the water- 

 flag.) 



This peculiar form of periwinkle, with its tentacle and eye-stalk 

 curiously united in one, and bearing the eye at its summit, was dis- 

 covered on the banks of the Thames, in the neighbourhood of Green- 

 wich, by Dr. Leach, in 1816. It abounds under stones, or in the mud, 

 or about the roots of water-flags, and it does not appear to have 

 been collected in any other locality. Forty years later, when a con- 

 troversy sprang up between Dr. Gray and Mr. Clarke in conse- 

 qiTence of the latter strangely asserting that the animal is a Trun- 

 catella, the same locality was had recourse to for living specimens. 

 Mr. Jeffreys, who undertook to procure them, wrote word to 

 Mr. Clarke : — " I went to Greenwich last Saturday, and have the 

 pleasure of sending you some lively examples of this curious mollusk 

 as well as a few Littorina (?) anatina. The shell of the latter is 

 closely allied to Bythinia, but the operculum is that of Littorina. I 

 found both of them more or less distributed along the banks of the 

 Thames, from a little below Greenwich Hospital to the upper pier at 

 Woolwich, a distance of about three miles. I met with them occa- 

 sionally in the same localities, but their habitats are somewhat 

 different. The Littorina {Bythinia similis, Dupuy, LLydrohia si- 

 inilis, Hartmann) inhabits muddy ditches and their banks, and it is 

 gregarious. The other mollusk (Assiminea Gray ana) inhabits muddy 

 places, but seldom occurs under water. It is in countless profusion 

 at and about the roots of the water-flag, and is more generally dis- 

 persed. It is associated with Limneus palustris and truncatulus." 



The shell of Assiminea Grayana is of a well-defined pyramidally 

 conical form, rather obtusely angled at the base. It is rather solid, 

 but yet translucent, of a reddish fawn colour. 



Family II. PERISTOMATA. 



Animal ivith the head produced into a ringed proboscis, carrying a tur- 

 binated shell with the aperture entire. 



The name Peristomata, signifying ' Entire-mouths,' was given 

 to this family by Lamarck, for the sake of distinguishing the oper- 

 culated freshwater Cephals, Bythinia, Paludina, and Valvata, in 

 which the shell is of a tubularly turbinated structure, and has the 



