TESTA discoidea, spird depressa^ apice semper 

 distincto^ anfractibus rotuiidatis^ plemmque ca- 

 rinulatis, aperturd Integra, altitudine latitudinem 

 ada3quaiite : labio iioiinuiiquam incrassato, ex- 

 pansOj inferiori parte prorsum extenso : umhilico 

 expansissimo. 



The principal peculiarity in this Genus, appears to us to 

 consist in the fact, that all the species, as far as we have had 

 the opportunity of examining- them, prove to be what are 

 commonly called reverse shells, that is, having- their volu- 

 tion turning- exactly in the opposite direction to that of 

 ordinary shells ; this fact has been doubted by some, while 

 others have either not observed or have denied it, and 

 consequently have described the principal species as being* 

 supra umbilicala. Fortunately for those who wish to be 

 correct, it is a point very easy to ascertain, as several of 

 the species are among- the commonest in stagnant waters 

 and ditches, almost every where ; so that whoever will 

 take the pains to observe, will find that the animals carry 

 their shells in an opposite direction to that of other testa- 

 ceous mollusca, and that the heart is placed in this on the 

 right side, and the respiratory orifice on the left, exactly 

 the reverse of their position in most others, but the know- 

 ledge of the animal is not indispensably necessary in proof 

 of this ; the shell possesses in itself all that is requisite for 

 the demonstration of this peculiarity. It is only needful 

 to observe, on which side of the shell the very apex of the 

 spire is actually to be seen, and taking- that side for the 

 upper, in conformity to the strict rules of analogy, it will 

 immediately be evident, that the aperture is on the left- 

 hand side : we are happy that an attentive examination, 

 not only of the animal inhabitants of several species, but 

 of the shells of several others, has enabled us to decide 

 this point to our own satisfaction ; because we are free to 

 confess, that for a long- time we had entertained great doubt 

 about the identity of some of the fossil species, which we 

 are now satisfied are reverse shells, in the same manner as 



