218 BRITISH PALAXOZOIC FOSSILS. [ Bracntopopa. 
cardinal area flat, low, triangular, ten times wider than high, inclining backwards at an angle of about 115°; 
internal casts of receiving valve minutely granulated and marked round the margin by sharp irregular traces of 
the external striz; muscular impressions extremely large, reaching slightly more than half the length of the 
shell, very coarsely suleated, forming a rounded, or broad ovate, patch without a defining lateral, or anterior, 
boundary, divided along the middle by an obscurely marked trace of a flattened internal ridge, terminating 
close to the beak in two short subparallel slits from a pair of small internal ridges; cardinal teeth very 
short, bifid, diverging at 125°, enclosing the triangular boss of the open foramen undivided from the flat cardinal 
area; entering valve flattened, with a narrow cardinal area rather less elevated than in the receiving valve ; 
interior large rostral tooth deeply bifid, forming two very large deep pits in the beak of the casts, exterior to 
which are the two short, narrow, faintly-impressed sulci of the small cardinal teeth, diverging at an angle of 100°; 
muscular impressions forming two obliquely-diverging narrow lobes, on each side of a thick short mesial ridge, 
extending little more than half their length; surface radiated with very fine, transversely-striated elevated lines, 
separated by still finer punctured sulci, the ridges divaricatingly arched towards the cardinal angles at the 
sides, very irregular in size, usually with the coarser about half a line apart, and with from one to three or five 
finer ones between each pair, the odd one after four or five lines becoming as large as the primary, and receiving 
fresh, smaller intermediate ones, so that the size and closeness of the striation is nearly uniform in all parts: 
there are about fifteen to nineteen striz in two lines at the margin of large specimens, but at six lines 
from the beak there are about twenty to twenty-five strize in two lines, about eight of which are larger than 
the rest, within an inch of the beak, but all becoming nearly equal towards the margin of larger examples ; 
hinge-line at the sides often marked with a few short oblique wrinkles; width one inch eight lines, proportional 
length =, depth 5). 
Some specimens of internal casts of receiving valve shew a subspinulose granulation, and two divaricating 
fan-shaped bundles of very slender, often-branched ovarian impressions, extending from the middle of the front 
of the great muscular impressions towards the front and sides of the anterior margins. The very large suite of 
specimens which I have carefully examined of casts of the receiving valve, shew in every case the triangular 
boss or impression of the foramen in the middle of the cardinal area, firmly united therewith to the apex, 
proving the foramen to have been open; for if it had been closed by a pseudo-deltidium, there would have been 
a vacant space left between the conical cast of the beak and the cardinal area; the species therefore cannot be a 
Strophomena, (in the sense of Mr Sharpe’s and this work,) but an extremely flat Orthis, a view also borne out by 
the size of the area in the entering valve. 
Position and Locality—Common in the Bala schists E. of Penmachno, Caernarvonshire ; very common 
in the fine Bala sandstone and limestone of Alt yr Anker, Meifod, Montgomeryshire; Bala schists of 
Llansantfraid, Glyn Ceiriog, Denbighshire; in ditto, Alt y Gader, near Llanfyllin, Montgomeryshire; Bala 
limestone of Llanfyllin, Montgomeryshire ; common in the Bala schists of Gelli Grin, Bala, Merionethshire : 
Bala schists of hills N. of Moel Uchlas, Montgomeryshire ; fine Bala sandy schists of Gaerfawr, Montgome- 
ryshire; fine sandstone of the Hollies, Church Stretton, Shropshire; Bala schists W. of Llanfechan, Mont- 
gomeryshire; Bala schists S. of Cwm Lanerch, Bettws, Caernarvonshire ; schists, Keeper’s Lodge, Golden- 
grove, Llandeilo; Bala schists of Das Eithin ridge, Hirnant, Montgomeryshire ; Bala limestone of Coniston, 
N. Laneashire; Bala limestone of Mathyrafal, Meifod, Montgomeryshire ; calcareous Bala schists of Llwyn y ci, 
N. W. of Bala; sandy schists of Middleton Park, Caermarthenshire; Bala limestone of Nant-y-arian, Llan- 
deilo; common in the Bala limestone of Llandeilo, Caermarthenshire ; calcareous Bala schists of Craig y 
Glyn, N. of Rhaider, near Llanarmonfach, EH. of Berwyns; abundant in, and almost composing the mass of the 
Caradoc limestone of Horderly. 
ORTHIS FLABELLULUM (Sov.) 
Ref.—Sow. Sil. Syst. t. 21. f. 8. 
Sp. Ch.—Transversely oval, or slightly subquadrate ; hinge-line slightly less than the width of the shell ; 
cardinal angles obtuse; receiving valve flat, or with a slight concavity half way between the beak and margin, 
with a high, triangular cardinal area at right angles to the plane of the margins of the valves (or not exceeding 
