JF. E. IIudleston—The Yorkshire Oolite. 121 



slioald be, is unfortunately not visible, and the base of the shell is 

 wholly obscured in matrix. 



Belations and Distribution. — Some might be disposed to regard the 

 differences between la and lb as important enough to constitute 

 two species. lb is an exceptionally depressed form, selected for 

 figuring as the best-preserved specimen which I could obtain. It 

 is entirely devoid of the undulations so conspicuous in la. But we 

 find shells with the outline and dimensions of la quite as devoid 

 of the undulations as in lb ; therefore this can hardly be regarded 

 as a character of importance, though D'Orbigny mentions it in his 

 diagnosis of Ditremaria amata. Again there is great variety in 

 the fineness of the ornamentation, but we cannot ti'ace a connexion 

 between any particular degree of fineness and the development of 

 these undulations. If any constant difference can be pointed out, 

 it is that the Grimston and Langton Wold shells are larger and 

 perhaps more depressed than those from the Eag of the Scarborough 

 district, which more resemble the small variety of this species 

 recognized by Morris and Lycett as rare in the Great Oolite of 

 Minchinhampton. 



Eoemer's shell, judging from his description, is a smaller and less 

 developed variety, with from two to three whorls. Brauns finds it 

 in the perarmatus beds of Heersum and Coralline Oolite of Hanover. 

 The species is not quoted from the Sequanien of Boulogne, nor do 

 I know it from the Coral Eag of the rest of England, though it 

 is decidedly common in the Coi'al Eag of Yorkshire. 

 Genus Pleurotomakia, Defrance, 1825. 



This genus is not particularly well represented in any member 

 of the Yorkshire Oolites, nor does the Inferior Oolite yield a better 

 list in this respect than subsequent formations. Here therefore we 

 note a great difference between the Inferior Oolite of Yorkshire 

 and of the Anglo-Norman basin, which latter may be regarded as 

 the head-quarters of Pleurotomaria, both in numbers and species. 

 On the other hand, the Corallian Eocks of Yorkshire have furnished 

 as many species as the beds of corresponding age throughout the 

 rest of England, though none of the species described can bo 

 regarded as common. 



50. — Pleurotomaria, sp. Plate IV. Figs. 2a, b, c. 

 Description. — Specimen from the Passage-beds of the Lower 

 Limestones, Wydale (my Collection). 



Length 21 -o millimetres. 



Width 22^ 



Spiral angle 74°. 



Shell conical, subturrited, but slightly oblique, scarcely umbilicated. 

 Spire composed of six or seven whorls, which increase regularly and 

 are but slightly angular. Each whorl has only one well-defined 

 keel, which forms the princij)al prominence, and carries the imbri- 

 cated sulcus. In the penultimate whorl of this specimen an anterior 

 keel is imperfectly developed. The ornaments display considerable 

 irregularity. Above the siilcus of the body-whorl the sculpture is 



