3ASTEROPODA. 539 



their systematic value is not uniform ; and therefore tha 

 attempts to remodel the arrangement of the Gasteropoda 

 by their peculiarities of dentition have not become so com- 

 plete a success as was at first expected. Some, how- 

 ever, hold a different opinion ; and Dr. J. E. Gray writes : 

 " One result of the study of these papers (Loven's, On 

 the Tongues of Mollusca) and the examination of the 

 tongues of several molluscs has been to establish more 

 firmly the theory which I have long entertained, that no 

 species of gasteropodus 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 attended with more or less uncer- 

 tainty." 1 



Dr. Troschel has laboured much in this field of investi- 

 gation, and in his valuable work on the subject attempts a 

 classification of the principal types by their lingual den- 

 tition. The union under one formula of so many creatures 

 widely differing in anatomy, habits, and shell structure, 

 clearly indicates that, if the lingual ribbon contains 

 generic characters, they have not yet been ascertained. At 

 the same time, it does present differences which may offer 

 collateral evidence in cases otherwise difficult of discrimi- 

 nation. It does not help us to separate carnivorous from 

 phytophagus animals ; but it seems possible to make use 

 of it as a mark between species ; for, in all, there is a dis- 

 tinct difference between the tongues even of the most 

 closely allied. Thus, amongst other changes, it has been 

 found necessary to remove the Proserpinadce from the 

 neighbourhood of the Gydophoridce, to which they were 

 formerly supposed to be nearly related, and to place them 

 in a more natural position near the Neritidce. That these 

 investigations are of value is also shown by the light 

 which has been shed on the true position of Aporrhais, 

 supposed by so great a naturalist as Forbes to be akin to 

 the Gerithiidce, but which is shown by its dentition to 

 belong to the Strombidce? 



(1) Annals of Nat. Hist. Ser. ii. vol. x. p. 413. 



(2) See a paper on the subject in the Trans. Linn. Soc. 1S67. 



