424 BRITISH PALALOZOIC FOSSILS. [Bracuropopa. 
point the profile arches abruptly to the apex of the much incurved beak, but slopes gradually with very slight 
convexity to the elliptical lobe terminating the mesial sinus, which projects, so that from the beak to its middle 
is considerably more than from thence to its lateral edge ; lateral edges only slightly convex ; cardinal area wide, 
moderately concave, subparallel-sided, abruptly tapering at the ends; mesial hollow wide, concave, bearing three 
equal obtuse ribs, the same size as the lateral ones till after half an inch from the beak, beyond which the outer 
pair bifureate, forming five (or sometimes, by trifureation of the outer ribs on each side, seven) ribs towards the 
margin in adult specimens, the two outer of which are less distinctly marked than the others; each side with 
from seven to fourteen rounded or obtusely angular ribs, usually simple, or one or two of them bifurcate at various 
distances from the beak; entering valve varying from triangular to semicircular, according to the width of the 
hinge-line and projection of the front lobe, moderately gibbous, becoming gradually flattened towards the 
cardinal angles; mesial ridge varying from triangular and half as high as wide, to flattened and scarcely 
elevated, its ridges less strongly defined than those of the sides or sinus, bearing three ridges close to the beak, 
which quickly become four by the bifurcation of the middle one till near the margin of large specimens, when 
the outer pair obscurely divide into two, or in very large specimens into three ; surface usually smooth, or crossed 
by delicate irregular lines of growth; but under a lens, when very finely preserved, reticulated. Average ordinary 
95 
width one inch four lines, proportional length of receiving valve varying from = to =, most usually 4, of 
entering valve from beak to central edge of mid-lobe 4, to front lateral margins =; depth of receiving valve *, 
of entering valve 2; cardinal angles commonly 70°, but varying from 55° to 85°; width of cardinal area * to 28. 
Occurs occasionally two inches eight lines wide, without further change of character. 
The var. rhomboidea Phill. agrees in ribbing, area, and every particular with the above ordinary types, but 
the width reaches the proportion of twice the length. The muscular impressions are small, and the dental 
lamellee scarcely reach one-fifth the length of receiving valve. I have not quoted Sowerby’s figure (Min. Con. 
t. 265), as it gives no definition to the mesial ridge, and makes the ribs so much broader than any of the great 
number of specimens I have examined, that it is scarcely recongnisable as a portrait of the ordinary forms. 
The lateral ribs in by far the greater number ef specimens are simple, and when one or two are branched on 
one side they often are not on the other. The remarkable projection in front of the mid-lobe does not occur in 
some specimens, otherwise similar. It will be seen that I find the common character of the ribs to be that 
supposed by Murchison, de Verneuil and Keyserling to characterise the S. Strangwaysi and S. incrassata, and 
not to occur in this species. From the traces of reticulation I have observed on those rarely seen spots where 
the surface is really perfect, I suspect the S. Wischerianus may be referred to this species perfectly preserved. 
Position and Locality —This variety is extremely common in the lower carboniferous limestone of Derby- 
shire; less so in the dark carboniferous limestone of Lowick; in the dark carboniferous limestone of Dent, 
Yorkshire; in the red carboniferous limestone of Breedon, Leicestershire ; dark lower carboniferous limestone 
of Berwick-on-T weed ; carboniferous limestone of Malham Moor; black carboniferous shale of Craige, Kilmar- 
nock; and in the limestone of same locality rare specimens, with the cardinal angles attenuated into a spine, 
(S. Strangwaysi M. V. K.); rare in the lower carboniferous limestone of Kendal; in the red lower carboniferous 
limestone of Closebourn, Dumfriesshire. 
Var. 8. BISULCATA (Sow.) 
Ref—Sow. Min. Con. t. 494. f. 1, 2; id. Geol. Trans. 2nd Series, Vol. V. t. 39. f. 21. 
Dese.—Subrhomboidal, hinge-line usually narrow, nearly parallel-sided ; beaks approximate, often nearly in 
contact ; cardinal angles only slightly acute; sinus in the front margin deep, and raised at right angles to the 
plane of the margins, rendering the mesial ridge shorter from the beak to its middle, than from the same point 
to its lateral front margins; lateral ribs obtusely rounded, usually simple, occasionally one or two of them dicho- 
tomous; the mesial ridges often obscurely defined, at first three near the beak, forming four by the bifurcation of 
the middle one, and towards the margin, after six lines from the beak, the outer one on each side branches, first 
into two, subsequently, in some specimens, into three ; mesial sinus with three ribs near the beak, each of which, 
towards the margin of old specimens, di- or tri-chotomises, giving rise to at most nine often obscure ribs, slightly 
