74 MOLLUSCA. 
aquatic testacea they are situate on the head, at the base 
of the tentacula; whereas the eyes in nearly all the terres- 
trial species are placed on the tipof these organs. We might 
also add, that no bivalve shells are found on the land; they 
belong exclusively to fresh water and to the sea. 
The fluviatile shells, though destined to reside in a differ- 
ent medium from the terrestrial, have in the greater number of 
genera their organs of respiration (accerding to Cuvier) near- 
ly the same, and are, therefore, compelled to come occasion- 
ally to the surface to respire. They have usually two flat ten- 
tacula, with the eyes placedatthe base. They may, in general, 
be distinguished from the marine kinds by the superior thick- 
ness of their epidermes, their corneous colour, and semi-trans- 
parency. 
The marine shells are the most numerous, the most beau- 
tiful, and the most highly prized of all the testacea. Many 
of the univalves of this tribe possess a lengthened respira- 
tory tube, with a canal in the shell for its protection, a cir- 
cumstance not observed in the fluviatile testacea. There is 
one circumstance which at once points out the difference in 
structure between the fluviatile and marine testacea: the 
fluviatile cannot live in salt water, nor the marine in fresh 
water. This fact points out an arrangement in their orga- 
nization to which conchologists ought to pay attention. 
These remarks are calculated to persuade conchologists 
to attend to the character furnished by the habitation of 
shells. In the formation of genera, it ought to be respected ; 
in the higher divisions it would be inconvenient. The care- 
Jessness of Linnzeus with regard to this character, is the prin- 
cipal reason why his genus helix is such a confused and in- 
digested mass. Were the distinction arising from habitation 
to be observed in the distribution of the testacea, no confu- 
