222 BRITISH PALALOZOIC FOSSILS. [ Bracutopropa. 
Yspatty Evan, N. Wales; calcareous Bala schists of Craig y Glyn, N. of Rhaider, near Llanarmonfach, E. of 
Berwyn Mountains; Upper Bala rock of Capel Cerrig; Bala schists of Llwyn y ci, Bala, Merionethshire ; 
schists right bank of Dee, Llangollen, Denbighshire; Bala schists of Conway Falls; Bala schists of Penlan, 
Llandovery ; Bala schists S. of Capel Garmon, Denbighshire; Bala schists of Llyn Ogwen, N. Wales; Bala 
schists of Goleugoed, Llandovery, Mandinan, Caermarthenshire; Bala limestone of Corwen, Merionethshire ; 
Bala schists of Dermydd Fawr, under Craig Bronbanog, Denbighshire ; var. in the Tremadoce slate of Tremadoe, 
Merionethshire ; ? var. in Caradoc sandstone of Horderly W.; ¢ var. Bala schists of Nant-y-arian, Llandeilo ; 
sandstone of Mulock Quarry, Dalquorhan; Bala schists of Tan y Bwlch y groes; var. avellana common in 
the Upper Bala limestone of Horton in Ribblesdale, Yorkshire. 
OrTHIS PLICATA (Sow. Sp.) 
Ref. and Syn.=Spirifer ? plicatus Sow. Sil. Syst. t. 21. f. 6. + Orthis virgata Sow. Sil. Syst. t. 20. f. 15. 
Sp. Ch.—Shell nearly semicircular or subquadrate ; hinge-line slightly exceeding the width of the shell; 
cardinal angles acute; both valves gently convex; greatest depth at one-third the length from the beak ; 
receiving valve deepest, with a moderately projecting beak, with a large flat triangular area, about one- 
fifth of the width high, inclining backwards at an angle of about 120°; surface radiated with very numerous 
subequal, angular, radiating ridges, separated by spaces slightly less than their thickness in width, increasing 
in number by occasional dichotomy, and by intercalation of new ridges, all nearly equal at the margin, 
where they usually amount to forty-eight or fifty, about five in the space of two lines at six lines from 
the beak, interstices marked with transverse strie ; entering valve with a triangular area about eight times 
wider than high, inclined at about 130° to the plane of the lateral margins: interior cast of entering valve 
shews the triangular boss of the opening in the area, divided by a slender longitudinal rostral tooth; the 
pits of the two cardinal teeth very small, rounded; a little in front of the hinge-teeth two obscurely-rounded 
projections of the muscular impressions, divided by a wide deep furrow, becoming obsolete before reaching 
the margin; interior cast of receiving valve with two slightly incurved, subparallel, dental lamellie, bor- 
dering the beak ; muscular impressions forming two large ovate patches, reaching more than half the length 
of the shell, separated by a wide space, divided by a very slender mesial septum, each impression sur- 
rounded by a delicate boundary, and filled with fine longitudinal furrows, branched at the end; in most 
casts a few broad branching pallial vessels approach the margin; the sharp subequal casts of the radiat- 
ing ridges extend to, or over, the muscular impressions in both valves, becoming gradually more prominent 
towards the margin. Width eleven lines, proportional length of receiving valve 7%, of entering valve 
depth about =. 
The outline varies considerably in width and acuteness of the cardinal angles, allowing for the difference 
apparent between Sowerby’s above quoted figures of the entering and receiving valves, to which he has 
given separate names, as was pointed out by Mr Salter (see note to page 287 of Memoirs of the Geol. 
Surv. Vol. IT.) The particular figure given as S. virgata presents an unusual size and distance of the 
main ribs, with small ones between, characters however which may be occasionally noticed in all parts of 
specimens of the ordinary type. The internal muscular and pallial impressions occasionally seen extending 
to the margin are usually effaced by the more or less distinct ribbing. 
Position and Locality—Common in the Bala schists of Gelli Grin, Bala, Merionethshire ; abundant in 
the Bala schists of Bryn Melyr, near Bala, Merionethshire; Bala schists of Llansantfraid, Glyn Ceiriog, 
Denbighshire; Bala schists of Cwm of the Cymmerig, E. of Bala, Merionethshire ; var. in the fine Bala sand- 
stone and limestone of Alt yr Anker, Meifod, Montgomeryshire; rare in the Bala limestone of Bryn Evan, 
Yspatty, N. Wales; Bala schists of Rhosfawr, N. of Glog, Llanfyllin, N. Wales; Bala schists of Das Eithin 
ridge, Hirnant, Montgomeryshire; Bala limestone of Meifod, Montgomeryshire; calcareous Bala schists of 
Bala, Merionethshire ; Bala schists of Mathyrafal, Meifod, Montgomeryshire; sandy Bala schists of Pwllheli, 
Caernarvonshire; Bala schists of Maes Meillion, S. of Bala, N. Wales; a fine ribbed var. in the Bala schists 
of Gaerfawr, Montgomeryshire; impure Pala limestone of Penlan, Llandovery; a wide, close-ribbed var. in 
calcareous Bala schists of Cader Dinmael, near Corwen, Denbighshire; Upper Bala limestone, Horton in 
Ribblesdale, Yorkshire. 
72 
100? 
