442 BRITISH PALAZOZOIC FOSSILS. [Bracuropopa. 
which they deeply reflexo-dentate; no lateral ribs. Surface smooth, or with obsolete, obtuse, thread-like strize, 
about eight in one line under the lens, crossed by similar obsolete striae of growth; shell tissue of moderately 
coarse flexuous fibres, Dental lamellze in receiving valve very slightly diverging, about one and half lines long ; 
mesial septum in entering valve about three and half lines long. Average width about one inch six lines, 
from beak of receiving valve to middle of ran sinus /;,, from beak of entering valve to same point 4, from beak 
of receiving valve to lateral front margins ;;, nuit of sinus 7, depth thereof about the same, but often less, 
depth of entering valve from middle of side margins ,;,, depth of receiving valve from same point 5}. 
This species is extremely constant in its characters, and certainly distinct from all the varieties of H. acu- 
minata, to which M. de Koninck unites it, by the remarkable character pointed out by Sowerby, of the inflation 
of the receiving valve, giving it the appearance of hanging much below the commissure. 
Position and Locality —Not uncommon in the carboniferous limestone of Derbyshire. 
Genus. PENTAMERUS (Som.) See page 208. 
PENTAMERUS CARBONARIUS (M°Coy). Pl. 3. D. figs. 12 to 18. 
Ref.—id. id. M*Coy, Annals of Nat. Hist. 2nd Series, Vol. X. 
Desc.— Globose, more or less inequilateral; hinge-line nearly as wide as the shell; cardinal angles obtuse, 
lateral and greater portion of front margins in one plane; middle of front margin abruptly raised into a very 
narrow, short, oblong or rounded sinus; commissure blunt from the meeting of the valves at a very obtuse angle. 
Entering valve varying from semicircular to rhombic, very gibbous; beak large, prominent; profile regularly 
arched from the apex to its front margin, its greatest depth about the middle of the length ; mesial ridge narrow, 
flattened, prominent, and strongly defined from the front margin to the apex of the beak, either simple, divided 
by one mesial hollow, or divided into four narrow ridges near the margin; each side with about seven very large, 
rugged, angular, irregular, subequal ridges, at six lines from the beak, beyond which they are either continued 
simple to the margin, or some or all of them dichotomise. Surface rather rugged, and very coarsely granulo- 
punctate, or minutely pustular under the lens, with strong thickened interruptions of growth at the margin, 
after nine or ten lines long. Receiving valve extremely gibbous ; beak very large, usually slightly inclined to one 
side, varying greatly in its inrolment, according to the form of the cardinal area, which is sometimes nearly 
half as high as wide, triangular, very slightly concave, and nearly at right angles with the plane of the lateral 
margins, in which case the beak is prominent, and only slightly incurved, the greatest depth of the valve being 
a little in front of its apex, and the profile arching very gradually from thence to the front margin; in other 
specimens the beak is incurved so as nearly to touch that of the entering valve, when the cardinal area is greatly 
reduced, very concave, and lying nearly in the plane of the lateral margins, the profile being more than semi- 
circularly curved; mesial hollow very strongly defined by two thick ridges from the apex to the narrow sinus in 
the front margin, either simply hollowed, or bearing four ridges, much smaller than the lateral ones; lateral 
ridges about ten or twelve on each side at seven or eight lines from the beak, beyond which they are either 
simple, or more frequently di- or tri-chotomised, as they approach the margin, more rugged and irregular in 
direction than those of the entering valve, varying from three to six in three lines at one inch from the beak, 
according to the amount of bifurcation; transverse irregularities of growth more rugged than in the entering 
valve. Internal mesial septum of receiving valve very long, reaching nearly to the front margin, the lines of 
growth slightly arched towards the beak; rostral divaricating portions scarcely two-thirds the length of the 
combined mesial portion, but of slightly greater width where they are in contact; two mesial septa of entering 
valve narrow, scarcely one-third the depth of the mesial septum of the receiving valve, anterior broad ends very 
obliquely truncated, but reaching rather less than half the length pls the valve. Average width one inch two 
lines; proportional length of entering valve i length of hinge-line 4, length of receiving valve varying from 3 
100 
to =, height of cardinal area varying from j= in the ee to #4 in the latter, width ce areal ridge and sinus 
0 
in the front margin >, depth of entering valve % to 4, depth of receiving valve 5, to 7. 
Some of the specimens of this species so nearly resemble Spirifcrs, that it was not until I made sections in 
yarious directions of several specimens, demonstrating the invariable presence of the two narrow, longitudinal, 
