Bracwropopa. ] UPPER PAL/ZOZOIC MOLLUSCA. 421 
obscure ribs, somewhat smaller than those of the sides; shell under a strong lens, with very minute flexuous 
fibres. Average width of adult specimens one inch three lines, proportional length of receiving valve * to 
iz» length of entering valve =, length of hinge-line 7, width of cardinal area =, width of sinus in front 
margin +, depth of ditto =, depth of entering valve 2, depth of receiving valve =. 
In some localities this species is extremely common, but on the other hand it is of local distribution. 
Two of the characters on which Mr Sowerby lays stress, viz., the narrow sulcus dividing the mesial ridge 
of the entering valve, and the absence of plaits in the mesial hollow, are by no means important; for speci- 
mens are almost as often found in which the mesial ridge is undivided, and nearly all specimens shew the 
ribs I mention in the mesial hollow, although they are often very faintly marked. Professor Phillips’s 
figure for this species appears to be rather the young of his S. rotundata, the S. subrotundata of this work. 
But even when specimens of the same size are compared, the S. pinguis may be distinguished from the latter 
species by the width very slightly or not at all exceeding the length, its much greater depth, inflated sides, and 
consequently obtuse edges. 
Position and Locality— Very common in the limestone of the Isle of Man. 
SPIRIFERA RECURVATA (de Kon.) 
Ref.—de Kon. Anim. Foss. Bel. t. 16. f. 5. 
Desc.—Longitudinally ovato-subtrigonal; moderately gibbous; hinge-line much shorter than the width 
of the shell; cardinal angles obtuse ; greatest width a little behind the middle, from whence the sides con- 
verge with little convexity to the narrow front, which has a very narrow, abrupt, semicircular wave in the 
middle; lateral margins very slightly sigmoidal. Entering valve evenly gibbous, greatest depth about one- 
fourth from the beak ; mesial ridge very narrow, slightly convex, very little elevated, strongly defined by a 
deep sulcus on each side, having at six lines from the beak about four ribs on the mesial ridge slightly smaller 
than those of the sides, about seventeen obtusely angular subequal ribs on each side, produced by one or 
two bifurcations in that length from the beak; receiving valve evenly gibbous in the middle, rapidly sloping 
to the sides, greatest depth at one-third from the beak, from whence the profile arches gradually to the 
front margin, and rapidly to the apex; beak large, prominent, much incurved ; cardinal area wide, triangular, 
about two-thirds wider than high; mesial sulcus very narrow, sharply defined from the apex of the beak, 
and containing about the same number of ribs as the opposite ridge. Length seven and half lines, propor- 
tional length of entering valve ;4, width of hinge-line =, width of sinus in front margin =, depth of en- 
tering valve =, depth of receiving valve 7. 
The numerous, often branched, subequal, lateral ribs, and very narrow defined ribbed sinus, as well as 
elongate form, easily distinguish this species from its congeners. From the bifurcation of the ribs M. de 
Koninck gives 8 or 9 ridges to the mesial fold, at 10 lines long, and 18 or 20 on each side. 
Position and Locality—Not very uncommon in the carboniferous limestone of Derbyshire. 
SPIRIFERA SEXRADIALIS (Phill.) 
Ref.—Phill. Geol. York. Vol. II. t. 10. f. 8. 
Desc.—Rotundato-quadrate, gibbous, slightly longer or shorter than wide; hinge line very short; cardinal 
angles much rounded; anterior lateral margins rather straight, front obscurely narrowed. Entering valve 
obscurely ovato-trigonal, moderately gibbous; mesial ridge round, large, prominent, notched in front by the 
abrupt elevation of a small semielliptical sinus equalling it in width, at right angles to the plane of the lateral 
margins; very strongly defined from the beak by a deep, wide, concave sulcus on each side; two slightly 
smaller, obtusely rounded ribs on each side, having a third imperfectly defined on each outer side near the 
cardinal angles, which are smooth, Receiving valve very gibbous, with a deep, narrow, mesial hollow, strongly 
defined from the apex, bounded on each side by a thick, obtusely rounded ridge, more prominent than the 
lateral ones, of which there are two rather small on each side, leaving nearly half of each side towards the 
