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RISSOA. Freminville & Desmarest * . 
Gen. Char. Shell univalved, spiral, oblong or tur- 
reted, not umbilicated, often costated ; aper- 
ture entire,, oval, oblique, dilated, rather an- 
gular above and with a slight sinus at the base ; 
lips nearly united, the outer thickened, its edge 
not reflected. Operculum shelly. 
Under this genus are included a number of small ele- 
gant marine Shells, many of which are ribbed, especially 
in the young state, while some are nearly plain ; the 
outer lip being thickened more than the cost®, but not 
producing varicose sutures, and the obscure sinus or 
truncation at its base define it neatly. 
This genus is only known in the great Oolite, the 
newer parts of the tertiary formation and recent. 
RISSOA iaevis. 
TAB. DCI X.—Jig. 1. 
Spec. Char. Oblong-oval, smooth. 
.The sides of the whorls are rather flat, and the aper- 
ture less expanded than in some species : the two last 
whorls are almost cylindrical. 
* Bull, de la Soc. Phil, de Paris, 1814 . 
