W. H. Eudleston—The Yorlcsliue Oolite. 123 



is accompanied by approximately identical modifications of form 

 and growth. 



51. — Pleurotomakia Munsteri, Eoemer, 1839. Not figured. 

 Fleurotomaria Milnsteri, Eoemer, 1839, Suppl. to Ool. Geb. p. 44, pi. xx. fig. 12. 

 Idem, D'Orbigny, 1852, Terr. Jurass. vol. ii. p. 549, pi. 416, 



figs. 4-8. 



There occurs in the Upper Limestones at Filey Brigg, and else- 

 where about the same horizon, a Pleurotomaria which, though not 

 much larger than the one previously described, approaches still more 

 closely to Eoemer's species in its outline, dimensions, and the step- 

 like character of the whorls, which has only one keel. In more 

 typical specimens of PI. Milnsteri, such as those from the Elsworth 

 Eock, and especially from the Trigonia-heds of Osmington (Corallian 

 Series of Weymouth), the transverse ribbing is more continuous and 

 less granulated than in the Yorkshire specimens. This arises from 

 the comparative shallowness of the decussating furrow, and is pro- 

 bably too much the result of varying conditions of preservation to be 

 regarded as a feature of primary importance. 



Though tolerably common, I have never been able to obtain a 

 specimen in good preservation, but there is a fossil in the Leckenby 

 Collection marked " Pleurotomai-ia granidata, Sow., Lower Calcareous 

 Grit, Filey Brigg," which I have very little doubt belongs to this 

 variety, and probably comes from the Upper Limestones of that 

 locality. 



52. — Pleurotomakia reticulata, Sowerby, 1821. Plate IV. 



Fig. 3. 

 Trochus reticulatus, Sowerby, 1821, Min. Conch, table 272, fig. 2, 



Bibliography, etc. — Sowerby's original specimens were from the 

 Lower Kimmeridge of Eingstead Bay, observed also at Portland 

 Ferry in the same formation. He gives as the diagnosis " conical, 

 transversely reticulate-striated ; whorls bicarinated, base convex." ^ 

 Description. — Specimen from the Coral Eag of Settrington (my 

 Collection). 



Length 33 millimetres. 



Width 31 „ 



Spiral angle ? 73°. 



Shell trochiform, turrited, but slightly oblique, scarcely umbili- 

 cated. Spire composed of six or seven whorls, which are strongly 

 bicarinated, the upper keel being more acute than the other. It is 

 situated at the angle of the whorl, and carries the band of the sinus. 

 The imbrications are smoothed hj attrition. The ornaments con- 

 sist of a system of transverse lines, met at right angles by a system 

 of longitudinal ones, thus producing a net-like or reticulate structure, 

 which appears, however, to be subject to considerable irregularity, 

 and sometimes approaches the granular ornamentation of the 



' Both the figure and description by Sowerby incUcate that this species should be 

 bicarinated. Yet in the type collection at the New Natural History Museum are 

 three specimens not figm-ed (the figured specimen should be at Cambridge), which 

 clearly come from the Triffonicf-htds of AVeymouth, and belong to its very close 

 relative, Fl. Milnsteri, the more usual Oxfordian form. 



