404 DISSIMILAR ANIMALS IN 



" A similar difficulty exists with regard to Siphonaria 

 and Ancylus, genera belonging to two 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 Siphonaria? ; 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 interrupt- 

 ed by the canal through which the air passes to the respira- 

 tory organs ; yet the animal of Ancylus has long tentacles, 

 and eyes placed as in the Lymnaeae, to which it is closely 

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

 respects agrees with the equally marine genus Amphibola, 

 confounded by Lamarck with the Ampullariae. 



"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 cha- 

 racter 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 

 Littorinae, and all the other animals of the order to which 

 they belong, have their eyes placed on small tubercles on 

 the outer side of the base of the tentacles, which are gene- 

 rally 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 hitherto unpublished work on British Mollusca, under the 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 Assiminia grayana ; and as this name has been re- 

 ferred to by Mr. Jeffries and other conchologists, it may be 

 regarded as established, and that of Syncera hepatica, pro- 

 posed by myself in the ' Medical Repository,' vol. x. p. 239, 

 will take the rank of a synonym. 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 4-fasciata and several others of the smaller Lit- 

 torinae, and had been figured in the Supplement to Wood's 

 Catalogue, t. vi. f. 28, under the name of Turbo francesiae. 



" Taking this in conjunction with the preceding, we have 

 here two instances of univalve shells apparently belonging 

 to the same genus, the one found in fresh and the other in 

 salt water, but proving, when their animals are examined, to 

 belong to genera essentially distinct. My next illustration 



