82 ]\f r Gray on Testaceous Mollusca, 



difficulty of distinguishing between genera of shells, inhabited 

 by very different animals. 



A similar difficulty exists with regard to Siphonaria and An- 

 cylus, genera belonging to different families, one inhabiting the 

 sea-shores, while the other lives in rivers and brooks. The only 

 distinction between the shells of these two genera consists in the 

 Ancyli being generally of a thinner substance than the Siphon- 

 aricB ; but this is by no means an adequate character, some 

 species of Siphonaria {S, Tristensis, for example), being quite 

 as thin in texture as any Ancylus, Both have the muscular 

 impression interrupted by the canal through which the air passes 

 to the respiratory organs ; yet the animal of Ancylus has long 

 tentacles, and eyes placed as in the LymncecB^ to which it is close- 

 ly allied, while Siphonaria has no distinct tentacles, and in these 

 respects agrees with the equally marine genus Amphibola, con- 

 founded by Lamarck with the AmpullaricB. 



About fifteen years since, I first observed, in the marshes near 

 the banks of the Thames, between Greenwich and Woolwich, 

 in company with species of Valvata, Bithynia, and Pisidium, 

 a small univalve shell, agreeing with the smaller species of the 

 littoral genus Littorina in every character both of shell and 

 operculum ; yet this very peculiar and apparently local species 

 has an animal which at once distinguishes it from the animal of 

 that genus, and from all other Ctenobranchous Mollusca. Its 

 tentacles are very short and thick, and have the eyes placed at 

 their tips ; while the Littorina, and all the other animals of the 

 order to which they belong, have their eyes placed on small tu- 

 bercles on the outer side of the base of the tentacles, which are 

 generally more or less elongated. The shell in question and its 

 animal were described and figured by Dr Leach, in his hitherto 

 unpublished work on British Mollusca, under the name of Assi- 

 minia Gray ana ; and as this name has been referred to by Mr 

 Jeffries and other conchologists, it may be regarded as establish- 

 ed, and that of Syncera hepatica^ proposed by myself in the 

 Medical Repository, vol. x. p. 239, will take the rank of a sy- 

 nonym. A second species of this genus has lately been made 

 known by Mr Benson, by whom it was found in ponds in India. 

 Its shell is banded like that of Littorina ^-^asciata and several 

 others of the smaller Littorina, and had been figured in the 



