398 GASTEROPODA OF THE INFERIOR OOLITE. 
334, PLEUROTOMARIA BICINGULATA, sp. nov. Plate XXXIII, fig. 3. 
Cf. Prevroromaria Maria, d’Orbigny. Terr. Jur., vol. ii, p. 480, pl. ccelvi, figs. I— 
11 (= Pl. decipiens, var. turrita, Deslong- 
champs). 
Description : 
Height : - . : Safin 
Basal diameter , ; 5 . ‘oi mm, 
Spiral angle 5 : =, AB: 
Shell acutely conical, not umbilicate. Spire regular to slightly concave. 
Whorls (eleven to twelve) flat; sutures close. The base of each whorl is marked 
by a slightly crenulated rim, which projects over the succeeding whorl, and in the 
more adult whorls becomes smooth ; spiral lines, slightly decussated, constitute 
the general ornamentation. 
The sinus-band is anterior and prominent in the later whorls, and the spiral or- 
nament is almost effaced ; a round and almost smooth belt, here and there marked 
by growth-lines, remains. This belt of the sinus-band and the basal rim together 
constitute a marked bicarination in the anterior half of the whorls. The body- 
whorl is narrow and angular at the periphery; base flat towards the margin, 
slightly excavated in the centre, spirally striated with faint decussation. Aperture 
subrhomboidal and depressed with a sharp columellar notch. 
Relations and Distribution.—This form differs from Pl. Marix chiefly in having 
the basal carine rather more prominent than the sinus-band, whereas in the 
species from the Lias the prominence is represented as being equal ; the spiral 
angle also is less concave. PI. bicingulata, through the prominence of the sinus- 
band, is related to Pl. punctata, whilst the presence of a modified basal rim 
connects it with certain varieties of the elongata-group, such as Pl. Pictaviensis. 
Typical specimens occur in the Concavus-bed, Bradford Abbas, where they 
seem to shade off into the narrow variety of Pl. elongata. In some specimens the 
whorls are so much undercut that the shell, if held with the point downwards, 
appears like a nest of funnels, one within the other. 
