Bracutopopa. | LOWER PALAXOZOIC MOLLUSOA. 213 
tooth, and two short, prominent, very divergent cardinal teeth, each with a cardinal pit between it and the 
hinge, and a thick, obscure, mesial ridge, with usually two rounded, subequal, depressed muscular impres- 
sions on each side, the tumid boundaries of which sometimes form a cross. 
The opening in area of the receiving valve being free, and without trace of pseudo-deltidium, distinguishes 
this genus perfectly from the others of the family; and in casts this produces a remarkable difference, the 
triangular rostral portion of the receiving valve being inseparably fixed to the flattened cardinal area, while 
in Strophomena the triangular beak is separately defined all round, on account of the closing of the opening. 
OrtHIs ACTONIZ (Sow.) 
Ref.—Sil. Syst. t. 20. £. 16. 
Sp. Ch.—Truncato-orbicular ; hinge-line equal to the width of the shell; cardinal angles acute; receiving 
valve very convex; entering valve flat; cardinal area in both valves low, nearly equal, slightly curved, both 
inclined inwards to the commissure, so that the beak of the convex receiving valve is nearly on a level with 
the lateral margins; valves radiated, with from eleven to twenty (in the latter case several ones on the ears 
abruptly smaller than the rest, total number most usually thirteen) large angular ribs, separated by angular 
hollows ; ribs usually simple for about six lines from the beak, when most of them split irregularly into two 
or three less deeply-defined branches; surface crossed by fine imbricating strie of growth; casts of the 
convex or receiving valve shew at the beak two short, thick, cardinal teeth, diverging at an angle of 95°; 
the surface nearly smooth, with narrow definite ribs, separated by broad flat spaces, a few at the middle, 
reaching quite to the beak, the rest nearly so, their marginal extremities abruptly branching two or three 
times, and with occasional, short, intervening ribs; cast of entering valve with the diverging pits, of two 
small cardinal teeth, nearly parallel with the hinge-line, from which they are separated by two small projections 
for the cardinal pits, and in the centre of the beaks a small longitudinal depression for the rostrum; in front of 
these is a shallow mesial depression, flanked by two obtuse, obscure, rounded elevations, marking one pair 
of muscular impressions, the ribs not reaching quite to the beak. Width of small specimen ten lines, propor- 
tional length = to 7%, depth ;% to ,{;, height of both areas 4. 
The entering valve seems concave in many specimens, but I think only from violence—the best 
specimens shew it flat. ‘The large few angular ribs separate this from O. calligramma and O. flabellulum, 
even when the characteristic branched extremities cannot be seen. 
Position and Locality —Extremely common in the fine sandstone of Acton Scott, Church Stretton, 
Shropshire ; Bala sandstone of Alt yr Anker, Meifod, Montgomeryshire; Bala schists of Gelli Grin, Bala, 
Merionethshire ; schists of Blaen y Cwm, W. of Nantyr, Glyn Ceiriog, S. of Llangollen, Denbighshire ; 
Bala limestone of Tyn y Cabled, Llanfyllin, N. Wales; Bala schists of Llansantfraid, Glyn Ceiriog, Denbigh- 
shire; Bala schists of Maes Meillion, S. of Bala, Merionethshire; Bala schists of Cader Dinmael, near 
Corwen; Bala schists of Bryn Melyn, near Bala Merionethshire; Bala schists of Pont y Glyn, Diffwys, 
W. of Corwen, Merionethshire; Ingleton and Thornton, Beck; Horton, in Ribblesdale, Yorkshire; Bala 
schists of Cwm of the Cymmerig ; Coniston (Bala) limestone, of Coniston Water-Head, N. Lancashire; Bala 
limestone of High Haume, Dalton in Furness; Bala schists of Rhiwargor, near Llanwddyn, Montgomery- 
shire; Bala schists of Bryn Eithin, Penmachno, N. Wales; one doubtful variety in the Bala schists of 
Mathyrafal, S. of Meifod, Montgomeryshire, resembling Sowerby’s figure of his O. flabellulum, var. 8 (Sil. 
Syst. t. 19. f. 8), but with only fourteen ribs before their increase from the middle of the shell. 
ORTHIS BILOBA (Linn. Sp.) 
Ref. and Syn.=Anomia biloba (Linn.) Syst. Nat. 1154= Terebratula sinuata Sow. Linn. Trans. Vol. XII. 
t. 28. f. 5, 6.=Delthyris cardiospermiformis Dal., = Spirifer sinuatus Sow. Sil. Syst. t. 13. f. 10. = Orthis 
biloba Davidson, Bull. de la Soc. Géol. de France, 2nd Series, Vol. V. t. 3. f. 18. 
Sp. Ch.—Longitudinally obcordate, or subtrigonal; front wide, and very deeply bilobed; beak large 
incurved ; cardinal area flattened triangular, twice as wide as high; hinge-line straight, less than the width 
