I>tif^'R. Gray on the Tmgues of Mollusca, 413 



or Pyrula reticulata, and MM. Eydoux and Souleyet (Voy. de 

 Bonite) have figured the tongue of Pyrula tuba and other marine 

 Mollusca ; and more lately Mr. Thomson (in the Annals and 

 Mag. Nat. Hist. 1851, vol. vii. p. 86. t. 3) has published a most 

 interesting account of the dentition of British Pulmonifera. 



Dr. Troschel in his system, for some reason which I cannot 

 understand, places the family Ampullariadce with Cyclostoma and 

 Helicina, among the operculated Pulmonifera ; the families Ancy- 

 loidea and Siphonariacea, which have distinct lungs and no gills, 

 with the plumose-gilled Pleurobranchidaa, characterizing the 

 order Monopleurobranchiata, in which he arranges them, as having 

 a plumose gill. In his former paper (Wiegmann^s Archiv, 1836, 

 277) he referred the genus Ancylus to the order Hypobranchia, 

 which is quite as remarkable, since that order is generally colli' 

 fined to the genus Phyllidia. ' 



After studying these papers and examining the tongue of 

 many specimens of some species of Mollusca, I am satisfied that 

 the tongue offers a very permanent character of the species, and 

 is very rarely liable to variation. Characters of such permanence 

 in the species aff'ord one of the best means to divide the species 

 into natural genera ; and when we consider the important func- 

 tion the teeth have to perform in the ceconomy of the animal, 

 one may be convinced that any important alteration in the form 

 or position of the teeth must be accompanied by some corre- 

 sponding peculiarity in the habit and manners of the animal ; 

 hence they must afford good characters to bring together the 

 genera into natural groups or families. To carry out these views 

 will require a very much more extended series of observations 

 on these organs than we at present possess, though we know 

 enough at present to show that an examination of the kind will 

 produce most extensive changes in our existing system, and ex- 

 plain many points which are now involved in much obscurity. 



One result of the study of these papers and the personal exa- 

 mination of the tongue of various molluscs has been, to esta- 

 blish more firmly the theory which I have long entertained, that 

 no species of gasteropodous molluscous animal can be properly 

 placed in the system unless we are enabled to examine the animal, 

 the shell, the operculum, and the structure of its tongue ; and as 

 none of these parts but the shell can be examined in the fossil 

 species, their position in the various genera must be always at- 

 tended with more or less uncertainty. 



I have repeatedly observed, that there are many genera of 

 Mollusca which cannot be distinguished by the examination of 

 the shell unless it is accompanied by the animal. There are 

 several genera of marine univalves so alike in form and character 

 of the mouth of the shell, that they cannot be distinguished from 



