RISSOA. 373 



the other for the ' planorbis,' which is equally a Rissoa of 

 another form ? 



Is it to be contended, that because an animal has an elon- 

 gated shell of twelve volutions, and another a discoidal one of 

 three, it cannot be a Rissoa, and that such a departure from 

 the type, in the shape and length of the shells, demands that 

 the genus Cerithium be applied to the one, and Skenea to the 

 other? I would ask, what is the classic number of volutions 

 which stamp the Rissoidean animal? If the hard parts are 

 the essence of the science, the organs of vitality become of 

 secondary importance, and conchology rears its head again. 

 If the shells are taken into consideration with the vital organs, 

 ought the discrepancy in the form of the hard parts of a 

 certain number of animals, or the identity of their organs, to 

 determine the necessary genera? 



It may be said that the so-called 6. reticulatum has a canal 

 at the base of the aperture : this is scarcely so ; it is a mere 

 contraction and attenuation at that part, giving an effuse 

 aspect. The mantle is even with the shell, without a canali- 

 culation. Many of the Rissoce have these parts quite as much 

 developed. Again, it is said that its operculum and that of 

 the so-called Skenea planorbis are suborbicular : I say, not 

 more so than some of the Rissocs', and both these animals 

 have very much the same paucispiral rapidly-increasing cha- 

 racter of the opercular increment as in the Littorina. I 

 think that the C. reticulatum and S. planorbis differ less from 

 the Rissoidean type, the parva, than any other of the Rissoas 

 admitted by authors into that genus. If these positions are 

 not admitted, we ought, to be consistent, to manufacture a 

 separate genus for every petty variation of each Rissoa, and 

 expunge the term ' species' from the molluscan vocabulary. 



* Discoidalis. 

 R. PLANORBIS, nobis; O. Fabricius. 



Skenea planorbis, Brit. Moll. iii. p. 156, pi. 74. f. 1/2, 3; (animal) 



pl.G.G. f.l. 

 Helix depressa, Montagu. 



Animal yellowish-white, with a smooth umbilicated discoid 



