209 



above with a lunate, simple tooth, and beneath with small cartila- 

 ginous hooks ; pulmonary orifice on the collar, near that of the 

 vent ; genitals on the same side, and separate, that of the male 

 near the tentaculum, of the female at the margin of the collar. 



Obs. Lister first separated these shells by placing them in a 

 distinct section of the fluviatile kinds. In 1756 Guettard charac- 

 terized the genus very accurately, both the animal and its shell, 

 and applied to it the name by which it is now universally known. 

 Adanson and Geoifroy afterwards adopted the genus, the former 

 under the name of Gorct. Although this natural genus appeared 

 to be thus firmly established, our great master Linn6 subsequently 

 placed the species in the genus Helix, without any distinction 

 whatever ; an arrangement, however, in which he was not followed 

 by any of the distinguished naturalists who succeeded him. 



Lamarck in his earlier works, placed the genus near Ampidaria, 

 deceived perhaps by the equivocal characters of A. cornu arietes, 

 which he supposed to be a Planorhis, but he afterwards referred 

 the genus to its true place in the family of Puhnonea aquatica, 

 next to Limneus. Like all the species of this family, the Planor- 

 his never reside in deep water, but frequent the shores where they 

 oan resort to the surface to inhale the air. They inhabit fresh 

 water and abound in various parts of the globe. 



The shell of Planorhis has the appearance of being sinistral, 

 and this character has been almost universally stated in the generic 

 definition ; several conchologists, however, are now of the opinion 

 that it is dextral, notwithstanding the sinistral form of the animal. 

 Des Moulins in the " Actes de la Soc. Linn, de Bordeaux," says 

 that " the shell of the Planorhis is essentially dextral." Deshayes 

 in his account of this genus says that it is only necessary to 

 examine the greater number of the species and to compare them 

 with the dextral Limneus and sinistral Physa, to be convinced, 

 that in the normal position, the shell of Planorhis is truly dex- 

 tral ; but that the animal is really sinistral ; and he thinks we 

 ought rather to admit that a sinistral animal has a dextral shell, 

 than that the aperture is not in the normal direction, correspond- 

 ing with that of all shells yet discovered ; and that there is an 

 evident contradiction between the animal and its shell, as is also 

 exhibited in the Haliotis and probably in Ancylus. 



The species are rather numerous, and Deshayes describes eleven 

 fossil species of the environs of Paris. 



