Mr. W. Clark on Ancylus oblongus and A. fluviatilis. 283 



less extended spire. The salivary glands are conspicuous, and 

 the nervous masses of the brain, as in Ancylus, consist of larger 

 and smaller ganglia. The muscular structure of the Limneada 

 offers no points for particular remark. 



Some naturalists, as already stated, have removed from An- 

 cylus the A. oblongus, their Velletia lacustris, on the grounds 

 of generic considerations ; vv^e think our present notes will 

 dissipate these views. They adduce as an argument for di- 

 stinction, independently of structural contour, that the woof or 

 histology of the lingual riband is very diffei'cnt in the two. 

 We do not dispute this point, but consider it of no importance 

 except as a specialty, and observe, that if such variations are 

 to be regarded as the elements of generic composition, we 

 feel confident that naturalists who adopt them will involve their 

 generic states in inextricable confusion. We have examined the 

 lingual ribands of many species, and found singular discrepan- 

 cies, not only in the species of a genus, but in the same species, 

 that have convinced us of the unfitness of such bases for generic 

 dispositions. It may be admitted that the dentition of the 

 Vertebrata has been of use, especially in fossil cases, as generic 

 aids, but there is little analogy between it and the tongues of 

 the Gasteropoda. 



In the many Rissoce we have examined, we have found the 

 persistent frame of the riband, independent of the uncini and 

 other accessories, vary. We believe the same may be said of 

 the other Gasteropodan groups ; at the same time we do not 

 deny that some of them show a rough similarity of conforma- 

 tion, but it is unsatisfactory, and inapplicable for stable and 

 decisive generic determination. 



The late Professor Forbes at one time thought favourably of 

 M. Loven's views, and sent nie a copy of the Swedish pamphlet, 

 requesting my opinion of the value of the gasteropodan lingual 

 riband. After a laborious and painful examination, in con- 

 sequence of the minuteness of the Rissoidean ribands, I replied, 

 that I did not think the employment of the tongue of the 

 Gasteropoda, as a means of a new generic distribution, would be 

 attended with valid results, and that I was bound to say, I 

 greatly feared, if adopted, it would prove a source of much 

 confusion and unnatural determination. Professor Forbes, in 

 reply, informed me that, in consequence of a severe attack of 

 fever, he could not for a week or two make any comments on 

 my communication, and the subject was never afterwards re- 

 newed. 



Though the learned Professor, in his excellent ' British 

 Mollusca,^ has frequently mentioned the lingual ribands, I 

 believe he never considered them as likelv to become the accre- 



