umbilicus, is thin and unprotected. This latter formation 

 is by far the most frequent, and leads to the conclusion that 

 the majority of these shells have their opercula horny. 



On the distinctions between this genus and Planorbis, 

 little need be said. The principal difference consists in the 

 latter havino- no operculum ; but another, and a very remark- 

 able one, (which seems to have escaped all writers,) is, that the 

 shells of the latter genus are destitute of any columella. The 

 Planorbis cornn-arietis of Lamarck, has been removed by 

 Mr. G. Sowerby to this genus. This shell, it is true, appears 

 to be intermediate between one and the other; but the only 

 affinity which it bears to Ampullaria, is in the oval form of 

 the aperture ; vv'hile it is allied to Planorbis by its discoid 

 form, want of the columella, and being universally described 

 as without an operculum : the preponderance of evidence is 

 clearly in favour of the situation originally assigned to it by 

 Lamarck. 



The characters, therefore, given to the genus AmpuUaria 

 by Mr. G. Sowerby, will be found incorrect. There 

 was no necessity for explaining, much less for alter- 

 ing, (in this instance,) the masterly definitions of Cuvier 

 and Lamarck. With regard to the second species given by 

 Mr. Sowerby to illustrate this genus, he is no less in error ; 

 for the real A. riigosa, of all authors, is a strikingly distinct 

 shell from that which he has fio;ured under this name. This 

 will be sufficiently obvious by referrnig to the figures either 

 of Lister, Chemnitz, or Lamarck. 



Having offered these remarks on a subject to which I 

 have paid some attention, I wish to refrain from pointedly 

 noticing other errors and misconceptions into which 

 Mr. G. Sowerby has fallen ; rather wishing that greater 

 experience, and more matured judgment, may lead him to 

 do this himself, prior to the publication of the system of 

 Conchology which he has announced. 



