412 GASTEROPODA OF THE INFERIOR OOLITE. 



referred to this form further differ from the typical PL sulcata in having the 

 sutures rather more canaliculate and the umbilicus narrower and deeper ; on the 

 whole, too, the shells have less ornament. In most of my specimens the sinus- 

 band is flat or countersunk, and the body-whorl relatively very large. 



This is to all appearance an obconical variety of PI. sulcata, though not 

 exactly PL obconica, Tawney. 



The figured specimen of Tawney's " obconica" which must of course be 

 regarded as the type, has the whorls distinctly angular, as in PI. unisulcata, 

 d'Orb. But on the very same tablet (in the Bristol Museum) is an obconical 

 specimen with rounded whorls, just like fig. 7 of the accompanying plate. It 

 seems only reasonable to suppose that Tawney included such a form in his 

 " obconica." Inferentialiy, therefore, this variety represents PL obconica, Tawney, 

 although the figured specimen is not exactly the same. 



Relations and Distribution. — In Dorset PL sulcata and the obconical variety 

 just described are essentially fossils of the Murchisonse-zone, being especially 

 abundant at Coker and the Irony Nodule-bed at Burton Bradstock ; rarer at 

 Bradford Abbas and Stoford. The exact horizon at Dundry is not known to me, 

 although there is good reason to believe that it is in the Murchisonse-zone. 



This species, in common with many other Inferior Oolite Pleurotomariae, 

 probably occurs in the Cotteswolds. Most specimens there are in the form of 

 casts ; hence the exact species is not easy to determine. 



349. Pleorotomaria (Leptomario) Ajax, (VOrbigny, 1850. Plate XXXV, figs. 8, 



8 a, 8 b ; and obconical variety, 

 figs. 9, 9 a. 



1850. Pleurotomakia Ajax, d'Orbigny. Prod., i, p. 268. 



1854. — — Terr. Jur., vol. ii, p. 484, pi. ccclxxxviii, 



figs. 1—5. 

 Description : 



Height . . . . .15 mm. 



Basal diameter . . . .23 mm. 



Spiral angle (convex) . . . . 100°. 



Shell heliciform, with a narrow umbilicus. Spire convex and depressed, apex 

 very obtuse. Whorls (about six) narrow, tumid, and regularly marked with fine 

 spiral striae ; these ;ire very faintly cross-hatched in the earlier whorls, otherwise 

 there is no axial ornamentation ; sutures canaliculate. 



The sinus-band is narrow and sunken, marked with fine lines or plain ; in the 



