406 GASTEROPODA OF THE INFERIOR OOLITE. 
and the very wide-angled and concave forms which it is convenient to focus under 
Pl. Bessina. 
Pl. subglabra is somewhat rare; the very smooth varieties come from the 
Humphriesianus-zone of the neighbourhood of Sherborne. 
341. Prevrotomaria Bessina, d’Orbigny, 1854 (= Pl. mutabilis, Deslongchamps, 
pars). Plate XXXIV, figs. 4 and 5. 
1854, PLEvRoToMARIA Bessina, d’Orbigny. Terr. Jur., vol. ii, p. 460, pl. eeelxxvi. 
1873. _- — — Tawney, Dundry Gasteropoda, p. 41 
(33). 
Syn. = MUTABILIS, Deslongchamps, var. pATULA. Vol. cit., p. 111, 
pl. x, fig. 12. 
Bibliography, §c.—Dr. Wright first noticed this as a British species from 
Dundry (vide Tawney, loc. cit.). It will be observed that none of our specimens 
exhibit the submarginal line in the base, which is delineated in the “ ‘Terrains 
Jurassiques.” Fortunately, Deslongchamps speaks of the “cordon marginal” as 
slightly marked or entirely disappearing ; whilst his figure of the var. patula does 
not show a trace of it. Hence the absence of this feature in our English 
specimens need cause no apprehension. 
Description : 
Height : ‘ : 5 . 50 mm. 
Basal diameter : : : 140) mim: 
Spiral angle . : : . 80°—85°. 
Shell acutely conical, scarcely umbilicated. Spire extremely concave (‘‘ comme 
un tort chinois”’). Whorls about eight or nine, flat or but slightly excavated, 
with regular spiral strie throughout. In the more typical form (fig. 4), which in 
England is rare, the basal rim is prominent and coarsely nodular. As a rule, in 
British specimens the basal rim is flattened and subnodular (fig. 5), whilst in all 
cases the basal rim of the body-whorl in the adult shell has a tendency to become 
smooth. Flanks richly sculptured. 
The sinus-band varies with the age of the whorls, being on the whole narrow, 
raised, and close to the basal rim. The body-whorl largely projects beyond the 
angle of the spire, and in the adult whorl the sinus-band is slightly flattened; the 
ornaments are rather smoother than in the whorls of the spire. Base flat to sub- 
convex, spirally striated, and with a small umbilical slit (not a true umbilicus). 
The aperture is subrhomboidal, oblique, and depressed, and the traces of the 
columellar notch seem entirely obliterated. 
