SPECIES.
0. Orbiculata. Shell subglobular ; spire not prominent, but more than convex ; lateral line somewhat convex ; whorls five, obsoletely striated across, regularly rounded, Colour pale, greenish, yellowish, or slightly tinted with reddish, particu larly on the body, and margined above by an obsolete whitd line, on the middle of the body a white vitta revolves, sometimes obscure or wanting; aperture acute above, regularly rounded at the base, and ex tending from the center of revolution or base of the column to an equidistance be tween the base and apex of the spire ; base of the columella slightly projecting into an obtuse angle ; exterior lip whit ish, reflected. Length one-fifth of an inch.
Inhabits East Florida.
Jour. Acad. Nat. Sciences, vol. i. p. 283.
Animal pale; rostrum and tentacula blackish, the latter with a white line ; eyes very black, elevated in form of a short tubercle ; length about equal to the breadth of the shell ; foot not broader than the body ; tail rounded, or somewhat acute ; operculum simple, not spiral, yel lowish brown, minutely granulated.
This species we found in great num bers on what are called Oystershell Ham mocks,• near the mouth of the river St.
John, East Florida, in company with Po lygyra septemvolva. When in motion, the tentacula are elevated and depressed al ternately, as if feeling the way.
This shell is certainly a Linnxan but according to the improvements which have been made in Conchology, since the time of the Swedish naturalist, by Mr. Lamarck, and other systematists, it is at once excluded from that genus and its congeners, by having but two tentacula, and by its operculated aperture ; with the genus Cyclostoma, as it now stands, our shell has more affinity than it has to any other, but a very distinct generic cha racter is observable in the aperture, which is not orbicular as in the Cyclostoma, but is almost semi-orbicular, greater in length than in breadth, and the lips widely disu nited. In addition to the characters usu ally given of the animal of Cyclostoma, Mr. Cuvier remarks, that the tentacula are terminated by obtuse tubercles ; no such appendages are annexed to the cor responding members of this animal. Upon these considerations, I have thought pro per to construct the present genus.