PLEUROTOMARIA. 391 



Family— PLE UROTOM ARllBM . 



"Shell nacreous, variable inform; whorls regular in their volutions, deflected 

 or unrolled, but always exhibiting a special streak or sinus-band, constituted by the 

 obliteration of a slit of the lip ; this slit is generally permanent on the last whorl, but 

 in some genera is partly closed, and the character of the perforation changed; 

 aperture oval, rounded, or angular ; lip thin; operculum circular, corneous, with a 

 central nucleus." — Fischer. 



In the Inferior Oolite of Britain two genera represent this family, viz. PI euro- 

 tomaria and Trochotoma. 



Genus — Pleurotomaria, Bef ranee, 1826. 



"Shell eery variable; trochiform, turbinate, discoidal, or globular, internally 

 nacreous ; last whorl furnished with a notch of greater or less length, slitting the lip, 

 and elsewhere prolonged by an obliterated band, bordered by one or two elevated lines; 

 the growth-lines of the ivhorls above and below converge towards this baud with 

 posterior inclination ; the strise of the sinus-band are slightly curved and convex on 

 the side of the spire ; aperture oval or subrhomboidal ; lip thin; operculum corneous, 

 subspirat, or multispiral." — Fischer. 



As far as the Jurassic rocks of this country are concerned, the genus Pleuro- 

 tomaria seems to have reached its maximum development in those districts of the 

 Inferior Oolite which are characterised by a Cephalopod facies. But its distribution 

 is singularly unequal. Thus in the Dorset-Somerset district nearly all the beds, 

 especially those towards the base of the Upper Division which more especially 

 represent the " Oolithe ferrugineuse " of Normandy, abound with the fossilised 

 remains of Pleurotomarix, often in an excellent state of preservation. Dundry 

 must be included in this category. In the Inferior Oolite of the Cotteswolds 

 PI eurotom arise are fairly numerous, but badly preserved, and consequently of 

 very little use to the palaeontologist. In the Bast Midland district the genus is 

 sparingly represented, being almost entirely confined to the Northampton Sand 

 and the lower beds of the Lincolnshire Limestone. From the rich shell-beds of 

 Weldon and Great Ponton, believed to be in the upper part of the latter forma- 

 tion, Pleurotomaria is almost entirely absent or represented by dwarfed forms. 1 



1 PI. reticulata, Deslong. (PL subreticulata, d'Orb.), is doubtfully quoted by Morris from 

 Ponton. 



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