256 
smooth, double keel, bounding the band of the sinus, and covered with numerous 
small arched scales. The upper keel being covered by the whorl of the 
spire the double keel is seen best on the last whorl, which is slightly depressed 
above. The upper surface is ornamented with numerous, radiating, slightly 
wavy strim, bifurcating frequently towards their exterior extremity. The 
strise of the first whorls of the spire are obliquely transverse and scarcely 
waved at all. The slit is deep, and occupies nearly one-fourth of the total 
length of the last whorl of the spire. The umbilicus is obliterated by a rather 
strong callosity. The aperture is large, transverse, and sub-rectangular. 
Dimensions. — This shell can reach a length of about two centimetres, 
and a breadth of twenty- six millimetres ; its spiral angle is about 86°. 
Delations and Differences . — This species evidently belongs to the same 
group as P. carinata and P. conica, Sowerby. Its shell, like theirs, is 
often marbled or entirely coloured black, but it is smaller. It differs from 
P. carinata by the absence of the umbilicus, and by the depth of the strise, 
and from P. conica by the greatness of its spiral angle, and by the suture of 
the spire, which is directly joined to the keel, wliile in P. conica it is at some 
distance from the keel. 
Dorizon and Localities. — This species was found by Sowerby in the 
Carboniferous Limestone of Derbyshire. I liave rarely seen it in that of 
Vise. Mr. Clark found a single specimen of it in a yellowish argillaceous 
limestone at Dugiiid’s Hill. 
Pleurotomama gemmijlifeba, J. Dhillips. 
PI. XXIII, Fig. 9. 
Dleurotoniaria gemmidifera, J. Phillips, 1836, Geol. Yorkshire, II, p. 227, pi. 15, fig. 18. 
,, ,, L. G. de Koninck, 1813, Descr. Anim. Foss. Terr. Carb. 
Belg., p. 370, pi. 31, fig. 7. 
This shell is of moderate size, subconical, broader than long, composed 
of five slightly-convex sj)iral vrliorls, separated from each other by a shallow, 
linear suture. Their surface is ornamented with small, longitudinal, parallel 
ribs, furnished with small, slightly imbricated granulations. The grooves 
separating these ribs are crossed by fine oblique striae of growth. The number 
of ribs varies with the age, and is increased progressively l^y the insertion of 
