398 GASTEROPODA OF THE INFERIOR OOLITE. 



334. Pleueotomaria bicingulata, sp. nov. Plate XXXIII, fig. 3. 



Cf. Pleurotomabia Marijj, d'Orhigny. Terr. Jur., vol. ii, p. 430, pi. ecclvi, figs. 9 — 



11 (= PI. decipiens, var. turrita, Deslong- 

 champs). 



Description : 



Height . . . . .37 mm. 



Basal diameter . . . .31 mm. 



Spiral angle .... 48°. 



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 columella!* notch. 



Relations and Distribution. — This form differs from PL M arise- chiefly in having 

 the basal carinas 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. 



