I 



AMPULLARIA. 



t)f Planorbis : however, we are eertain tliat the principal 

 differences between the shells of these two Genera are to 

 be found in the form of the aperture, and in the presence 

 or absence of an operculum ; for which reason we are 

 obliged to take the shell he has given as the type of 

 Planorbis^ and add it to the genuine Ampullarice ; and this 

 we find the less difficulty in doing, as we possess a series of 

 specimens of decided Ampullariae, which pass, by regular 

 gradations, from an extremely ventricose, and nearly 

 globose, to as nearly a discoid form, without any material 

 alteration in the form of the aperture. We do not mean 

 to place any reliance on the fact, that all the species of 

 Planorbis, of which we have been able to examine the 

 living inhabitant, prove to be constantly reverse shells, 

 because we are aware that some of the Ampullariae are 

 also reverse ; but we are disposed to unite with Cuvier in 

 expressing our surprize, that a fact of so great import- 

 ance should either have been entirely unobserved or remain 

 unmentioned by some, while others (not satisfied v/ith dis* 

 believing the fact) maintain a contrary opinion. 



The true AmpullaricE may therefore be either nearly 

 globular with a more or less produced spire, or it may be 

 discoid, but still having a more distinctly elevated spire 

 than Planorbis: its whorls are ventricose, and it is umbili- 

 cated ; of course in the more discoid species the umbilicus > 

 is larger and more expanded, so that the whorls may be 

 counted on that side as well as on the side of the spire : 

 the aperture is oblongs entire and longer or higher than it 

 is wide, whereas in Planorbis it is on the contrary rather 

 the reverse ; in Ampullaria a shelly annular operculum, with 

 a nearly central nucleus, placed towards the inner side, and 

 coated by an olive green horny epidermis, exactly similar to 

 that on the shell, closes the aperture ; it is remarkable that 

 this operculum is subject also to the same kind of erosion, 

 commencing near its centre, which is so commonly seen near 

 the apex and on the prominent parts of the shell itself: 

 we think this rather confirmative evidence of an opinion 

 which has been entertained by some, but combated by 

 Lamarck, of the analogy of the operculum in these shells 

 to the second valve of bivalves. Without, however, pre- 

 suming ourselves to venture an opinion on so difficult a 

 subject, we wish to drav/ the attention of Conchologists 

 to the nature, use, and peculiarities of structure observable 

 in opeixula. 



