932 BRITISH PALAXOZOIC FOSSILS. [Bracuropopa. 
area. Our specimens correspond precisely with Pander’s type-figure above quoted, but if, as M. de Verneuil 
supposes, the various other species, on that and the succeeding plate be only varieties, it is obvious that 
there is considerable variety in the depth of the valves, and that the cardinal area does not invariably 
lean forwards to the extent mentioned above, but has sometimes even a slight backward inclination, as in 
the Orthidw generally; and in this case I think there could be no doubt that the whole might be well 
referred to the O. zonata of Dalman; but as I have not examined that species, I am unwilling (like M. 
de Verneuil) to make an uncertain approximation. 
Position and Locality—Not uncommon in the Bala slates of Cefn Coedog, Corwen, N. Wales; rare in 
the Bala schists of Cyrn y Brain, W. of Wrexham, Denbighshire. 
OrtuHisina Scorica (M‘Coy). Pl. 1. H. fig. 29. 
Ref.—ld. M°Coy, Annals Nat. Hist. 2nd Series, Vol. VIII. p. 400. 
Sp. Oh.—Subquadrate, valves evenly convex, much depressed; hinge-line as wide as the shell; apical 
angle 120°; sides subparallel, front with an obsolete emargination in the middle; entering valve gently 
convex, with an obscure narrow mesial depression from the beak to the margin; receiving valve with pro- 
jecting beak, not incurved, profile consequently nearly straight, very obscurely subcarinate along the middle, 
sides gradually sloping to the margins; cardinal area very narrow, parallel-sided in the entering valve, wide, 
flat, triangular and inclining backwards at only 120° in the receiving valve; pseudo-deltidium and deltidial 
pit concave, foramen oval, near the apex: surface radiated with twenty-eight thick, simple, subequal 
ridges, separated by deep sulci, rather less than the ridges in width (three in two lines at six lines from 
the beak in the middle). Width one inch, proportional length of receiving valve =, length of entering 
valve =, greatest depth of both valves (about the middle) ;;. 
There are two or three strong imbrications of growth at the margin, but I cannot distinguish any 
superficial strie in the specimens at my command. In form it is intermediate between the O. inflexa 
and the O. plana (Pander Sp.), but is more depressed, the beak of the receiving valve not at all incurved, 
the pseudo-deltidium concave outwardly, and is easily distinguished by its comparatively few, thick, 
simple ribs. 
Position and Locality—Very common in the concretionary limestone of Craig Head, near Girvan, 
Ayrshire; calcareous shales of Colmonel on the Stinchar. 
Explanation of Figures —P\. 1. H. fig. 29. Natural size from the limestone of Craig Head; 29 a, 
profile; 294, end view. 
Genus. LEPTAINA (Dal.) 
Syn. = (Leptena restricted + Strophomena Raf. + Leptagonia M°Coy + Chonetes Fisch.) 
Gen. Char.—Semicircular, depressed, entering valve concave, the other evenly convex from the hinge- 
line, which is as long as the shell is wide; cardinal area distinct, slightly rhomboidal, about equally 
formed of both valves; the convex or receiving valve has a triangular opening nearly filled by the project- 
ing angle of a rhomboidal, two or four-lobed, boss at the beak of the entering valve, leaving only a minute 
perforation, separated by a small pseudo-deltidium from the apex; punctured structure minute, often indistinct 
or obsolete: internal surface rough with little points; entering valve with a small, bifid tooth at the beak, 
in front of which are two small diverging ridges, between which is a moderate longitudinal septum. Some 
species shew a minute tubular foramen at the apex of the beak. 
This genus may be said to embrace four sections or subgenera not worthy I think of generic rank, 
on careful study of a very extensive suite of specimens of most of the known species. These are :—Ist, Lep- 
tena Dalman, restricted to the type of his last species the L. transversalis, in which the valves are almost 
equally curved in the same direction, the receiving or foraminated one convex, the other concave outwardly. 
2nd, Strophomena, in which the valves are flat or very slightly convex when young, the margin in a few species 
