CONCHOLOGY, 



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 septemvotva. When in motion, the 

 tentacula are elevated and depressed al- 

 ternately, as if feeling the way. 



This shell is certainly a Linnaean Helix, 

 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- 

 respondingmembersofthisanimal. Upon 

 these considerations, I have thought pro- 

 per to construct the present genus. 



GEXUS PLANORBIS. 



Shell discoidal; spire depressed, or 

 concave; aperture oblique, rounded, 

 broader than long, visible from above, and 

 emarginated by the convexity of the pe- 

 nultimate whorl : lips not reflected ; 

 whorls lateral. 



Animal aquatic, with two filiform ten- 



tacula, having the eyes placed at the inner 

 base ; operculum none. 



Observ. The species for which this 

 genus was constructed were included 

 by Linnaeus in his Genus Helix. The 

 spire is sometimes profoundly sunk, so 

 much so as to be with difficulty distin- 

 guished from the base. 



1. P. Trivolvis. Shell sinistral pale 

 yellow, brownish or chesnut colour, sub- 

 carinate above and beneath, particularly 

 in the young shell ; whorls three or four, 

 striate across with fine, raised, equi-dis- 

 tant, acute lines, forming grooves between 

 them. Spire concave ; aperture large, 

 embracing a considerable portion of the 

 body whorl, within bluish white ; lip a lit- 

 tle thickened, internally, and of a red or 

 brownish colour, vaulted above ; umbili- 

 cus large, exhibiting the volutions. 



Length one-fourth of an inch; breadth 

 one-half of an inch. 



Animal aquatic, dark ferruginous, with 

 very numerous, confluent, pale yellowish 

 points; tentacula long, setaceous, with 

 confluent points ; foramen on the left 

 side. 



That ingenious naturalist, Mr. C. A. 

 Lesueur, found this species of a much 

 larger size in French Creek, near Lake 

 Erie ; breadth three fourths of an inch 

 nearly ; colour almost black, purplish red 

 within the mouth. 



Plate 2. fig. 2. 



Cochlea, trium orbium. Lister. Conch. 

 tab. 140. fig. 46. 



Lister figures this shell pretty accu- 

 rately, and it is referred to in Gmelin's 

 Edit, of Syst. Nat. p. 3615, as Albella, but 

 it is certainly not that species. 



2. P. Bicarinatus. Shell sinistral, pale 

 yellow or brownish, subcarinate above, 

 and beneath translucent. Spire retus- 

 umbilicate, forming a cavity as deep as 

 that of the base. Aperture large, em- 

 bracing a considerable portion of the 

 body whorl, and much vaulted above, 

 Within red brown, with two white lines 

 corresponding with the carina. Whorls 

 three, wrinkled, and with minute revolv- 

 ing lines. 



Length one-fourth of an inch ; breadth 

 nearly half an inch. 



These are elevated knolls of oyster-shells mixed with earth, which rise, by an abrupt acclivity on all sides. 

 Irom the salt marshes in that conntry, to the elevation of fifteen or twenty feet ; they exhibit to the eye the 

 appearance ot old oyster beds, (Oyster Rocks.) which, owing to their compactness, have resisted the action of 

 the waters tor centuries, while the more yielding earth around them has been washed away to its present level. 

 by imperceptibleilegrees. 



XBT* 



