RISSOINA. 271 
Family—RISSOMD A. 
* Shell usually small, twrbinate; aperture oval or semilunar, entire or sub- 
canaliculate at the base, peristome continuous ; operculum horny.’—Fiscnmr. 
Genus—Rissoa, Fréminville, 1814. 
* Shell imperforate or subperforate, more or less oblong, turbinate; aperture 
scarcely oblique, oval, entire ; peristome simple or reflected.” —Fiscuer. 
This genus can scarcely be regarded as occurring in the Inferior Oolite, 
although ? Rissoa levis, Sow., has been quoted from the Cotteswolds by Lycett 
(Proc. Cottesw. Nat. Club, vol. i, p. 72). There is a smooth species found in the 
Lincolnshire Limestone at Weldon, which might at first sight be taken for Rissoa 
levis, Sow., but which has been described under Rissoina. 
Genus—Russoina, @ Orbigny, 1840 ?. 
* Shell turrited, whorls numerous, but slightly convex ; apea mammillated ; aper- 
twre oval, semilunar ; lip thickened internally, slightly reflected, canaliculate or sub- 
canaliculate in front ; operculum horny.’’—Fiscuer. 
The distribution of Rissoina in the Inferior Oolite is very local, and mainly con- 
fined to the upper beds of the Lincolnshire Limestone. A few obscure fragments 
have been found in the Lower Limestone (Witchell) of the Cotteswolds, and 
better preserved specimens are occasionally obtained from the Pea-grit. These 
are the earliest recognisable forms hitherto noted from the British Jurassics. 
The prevailing form of Rissoina in the Lincolnshire Limestone is R. obliquata 
Sow., or some of its varieties. This form Mons. Cossmann unites with FR. acuta, 
Sow., retaining the name acuta for the species. There are specimens both from 
the Pea-grit of the Cotteswolds, and also from the Lincolnshire Limestone at 
Weldon, which answer fairly well to Sowerby’s description of Lt. acuta, having the 
slender spire and straight ribs of that species, but they seem to run into R, 
obliquata, which is certainly the more abundant form. This latter may be divided 
into more than one variety. Taking this view of the case, I recognise three 
