PLEUROTOMARIA. 411 



Bibliography, Sfc. — Sowerby's type is at the Bristol Museum, and in all 

 respects corresponds with the specimen now figured. The specimen in the 

 collection of Sowerby's types at the British Museum seems to represent a 

 somewhat narrow variety of PL unisulcata, d'Orbigny. As pointed out by Tawney, 

 the whorls of Sowerby's species are convex but not angular. This may be gathered 

 both from the description and figure in the ' Mineral Conchology.' The entire 

 section (Leptomaria) of which PL sulcata may be regarded as the type-species, as 

 developed in the Inferior Oolite, presents a series of forms which run into each 

 other. Moreover, in such slightly sculptured shells what little ornament there 

 may have been is often modified by fossilisation, whilst the spiral angle varies so 

 much that it almost ceases to be a guide. Hence the division into " species " is 

 attended with unusual difficulty. 

 Description : 



Height . . . . .22 mm. 



Basal diameter . . . .32 mm. 



Spiral angle (convex) .... 95°. 



Shell heliciform and largely umbilicated. Spire convex, with an obtuse apex. 

 The spire-whorls (about six) are narrow, convex or scarcely augulated, and 

 separated by a suture, which in the early stages is somewhat canaliculate. The 

 ornaments are faintly impressed, and vary much ; specimens from Dundry 

 generally show radial furrows in the upper part of the whorls, whilst the visible 

 spiral ornamentation is chiefly anterior. Specimens from the Murchisonse-zoJie of 

 Burton Bradstock exhibit spiral strise on the spire-whorls throughout, and this, 

 in the apical portion, is reticulate, whilst the body-whorl is smooth, merely 

 showing the growth-lines. 



The sinus-band is antero-mesial and very narrow, being situated on the most 

 prominent part of the whorl ; it is usually countersunk and smooth. The body- 

 whorl is relatively large and smooth, the flexuous growth-lines constituting the 

 only ornament ; these are continued in the rounded base to the edge of the wide 

 and deep umbilicus. Aperture subovate and oblique. 



Obconical variety (figs. 7, 7 a, 7 b). 



Cf. Pleurotomabia obconica, Tawney. Dundry Gasteropoda, p. 45 (37), 



pi. iii, fig. 6. 

 Cf. also Pleurotomaria l^vigata, Deslonqchamps. Vol. cifc., p. 138, pi. xvii, fig. 7. 



The general outline is that of a blunt cone, the spire being convex and the 

 spiral angle ranging from 80° to 85°. Most of the specimens which I have 



