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? Ifthe 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 C. 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 Rissoe 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 Rissoe; and both these animals 
have very much the same paucispiral rapidly-increasing cha- 
racter of the opercular increment as in the Littorine. I 
think that the C. reticulatum and S. planorbis differ less from 
the Rissoidean type, the parva, than any other of the Rissoe 
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, pl. 74. f.1, 2,3; (animal) 
pLiG:G-th I. 
Helix depressa, Montagu. 
Animal yellowish-white, with a smooth umbilicated discoid 
