PLEUROTOMARIA. 391 
Family—PLEUROTOMARIIDA. 
“ Shell nacreous, variable in form ; 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. Plewro- 
tomaria and Trochotoma. 
Genus—Pinurotomaria, Defrance, 1826. 
* Shell very variable; trochiform, turbinate, discoidal, or globular, internally 
nacreous ; last whorl furnished with a noteh 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 whorls above and below converge towards this band with 
posterior inclination ; the strie of the sinus-band are slightly curved and convex on 
the side of the spire ; aperture oval or subrhomboidal ; lip thin; operculum corneous, 
subspiral, or multispiral.” —FiscHEr. 
As far as the Jurassic rocks of this country are concerned, the genus Plewro- 
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 Plewrotomarix, often in an excellent state of preservation. Dundry 
must be included in this category. In the Inferior Oolite of the Cotteswolds 
Pleurotomariex are fairly numerous, but badly preserved, and consequently of 
very little use to the paleontologist. In the East 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 Pl. reticulata, Deslong. (Pl. subreticulata, d’Orb.), is doubtfully quoted by Morris from 
Ponton. 
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