Bracuropopa. } LOWER PALOZOIC MOLLUSCA. 915 
This species is excellently figured and described by MM. Murchison, de Verneuil, and Keyserling, in 
their Geology of Russia, but united with the old O. flabellulum of Sowerby, from which it is constantly 
distinguished by its more regular definite ribbing, and the fine longitudinal striz in the interspaces, without 
short irregular ribs, the sharp extension of the ridges more nearly to the beak on the casts, the greater incli- 
nation of the large cardinal area, and its being situated in the most convex instead of the flat valve. 
The 0. Davidsoni (Vern.) of the Wenlock rocks, is distinguished by its area forming a considerable angle 
with the plane of the lateral margins, thus elevating the beak. 
Position and Locality—Very common in the sandstone of Tan y Craig, Builth; and sandstone of 
Goldengrove, Llandeilo; Bala schists of Blaen y Cwm, W. of Nantyr; Glyn Ceiriog, S. of Llangollen, Denbigh- 
shire ; Bala schists of Llansantfraid, Glyn Ceiriog, Denbighshire ; sandy schists of the Hollies, Church Stretton, 
Shropshire; a small variety, about seven lines wide and with sixteen or eighteen ribs, very abundant in the 
sandstone of Cerrig Cregyn, Anglesea, agreeing in striation and all other characters with the true type; rare 
in the Bala limestone of Coniston Water-Head, Lancashire; in the grey quartzite of Carn Goran, Cornwall ; 
yar. not uncommon in the Bala limestone, Horton in Ribblesdale, Yorkshire. : 
Var. @ CALLIPTYCHA (JM°Coy). 
A beautiful variety, agreeing in the character of the area with the regular type, but rather more convex 
(depth of receiving valve &), with sixteen or eighteen ribs, in the interspaces between which the striz are of 
unusual strength, wire-like, and also occurring on the ribs themselves near the margin. 
Position and Locality.—Rare in the Bala limestone of Llansantfraid, Glyn Ceiriog, Denbighshire. 
ORTHIS CONFINIS (Salt.) 
Ref—Salter, Quart. Geol. Journ. Vol. V. t. 1. f. 4. 
Sp. Ch.—Subquadrate, depressed, receiving valve with a slightly projecting conoidal beak, from which 
a slightly marked longitudinal keel, or convexity, extends for a few lines towards the margin, and from which 
the surface slopes with little curvature in all directions towards the margin, the greatest depth being at 
the beak ; margins in one plane, or in young specimens, with a minute, obtusely angular wave, in the front 
margin towards the receiving valve; entering valve gently convex, greatest depth about the middle of the 
length, with a very small depressed beak, from which a rather deep, narrow, undefined sinus extends for about 
four or five lines towards the margin, then gradually disappearing ; cardinal area curved, triangular, six times 
wider than high in the receiving valve, with a narrow, triangular, open foramen, inclining backwards at 100°; 
area of entering valve rather less than half the width of that in the receiving valve, to which it is at right angles ; 
both valves radiated with narrow, round, straight, prominent, very sharply-defined ridges, separated by flat, 
smooth spaces, as wide or considerably wider than the ridges, and increasing irregularly in number by inter- 
ealation at a short distance from the beak, in well-developed specimens, in which at six lines from the beak there 
are five subequal ridges in two lines; in other specimens the ridges are closer, every third or fifth stronger than 
the others, producing a fasciculation, and seven in two lines, ridges much closer on the sides than towards 
the middle. Width one inch one line, proportional length of receiving valve =, of entering valve ;;, 
depth =. 
The great difference which I have above registered in the number and general character of the radiating 
ridges, may be traced in such a way among the Scotch specimens as leaves no doubt of the specific iden- 
tity of the forms, some of the specimens, with more numerous ridges, having them nearly uniform in size, 
while some of the specimens with the more distant ridges have them more or less fasciculated, as in the extreme 
forms above alluded to; in all cases, however, the species seems very distinct, by the narrow, very sharply- 
defined, round linear ridges, and smooth, flat, wide interspaces; the casts are strongly marked with radiating 
ridges round the margin. 
Position and Locality—Extremely common in the impure concretionary limestone of Bugon, Knock- 
