214 BRITISH PALALOZOIC FOSSILS. [Bracuropopa. 
of the shell, ears slightly flattened, rectangular, sides slightly convex, diverging at a variable angle, front divided 
into two narrow rounded lobes, from which a deep angular sinus extends to the beak in each valve ; both valves 
moderately tumid, surface radiated with strong, sharply-marked, obtusely angular, irregular, dichotomous 
ridges (five in one line at the margin). Length three lines, proportional width at front =, length of 
hinge-line =, depth of both valves about =. 
The above-quoted figure of Mr Davidson shews the interior, with the rostral tooth flanked by two long, 
diverging, cardinal teeth, the margin strongly crenulated, and the two pairs of muscular impressions narrow, 
oblong, and obliquely elongated to two-thirds the length of the shell. 
Position and Locality—Calcareous Upper Bala schists of Cefn Coch, Glyn Ceiriog, Denbighshire. 
OrtTHIS CALLACTIS (Dal. 2.) 
Ref.—His. Leth. Suec. t. 20. f. 9. Sow. Sil. Syst. t. 19. f. 5. 
Sp. Ch.—Truncato-orbicular, both valves greatly depressed; hinge-line slightly exceeding the width of 
the shell; surface radiated with from sixteen to twenty radiating ribs, separated by broad concave spaces 
(forming very broad, rounded, smooth ribs in the interior) ; in most specimens the ribs are more or less divided 
into three by a pair of minor lateral furrows, reaching half way to the beak; cardinal area narrow in both valves, 
the entering valve with a slight concavity near the beak ; receiving valve with a slight convexity near the beak ; 
hinge-teeth of entering valve nearly parallel with the hinge-line, with a thick mesial ridge extending from 
between them nearly to the margin (coinciding with one of the external intercostal spaces). Width eleven 
lines, proportional length =, depth =. 
There is a great contrast between the interior and exterior views of this species, the former being 
radiated with very wide, obtusely-rounded ribs (agreeing with Hisinger’s figure), the exterior shewing narrow 
rounded ridges separated by wide concavities. The variety is extremely common and irregularly developed, 
in which the lateral sulci seem to render several of the ribs trifid, increasing the number and decreasing the size, 
particularly near the cardinal angles, and the margins of large specimens. The great depression of both 
valves easily separates the species from the O. Actoniw, to which it is most allied. On the interior, three of the 
middle ribs often appear to converge before reaching the beak. 
Position and Locality —Rare in the Bala schists of Bala, Merionethshire; more common and of larger 
size in the Bala limestone of Gaerfawr, Welchpool, Montgomeryshire; very common and still larger (often 
an inch long) in the limestone of Wray Quarry, Upper Tweed, near Broughton. 
ORTHIS CALLIGRAMMA (Dal. as defined by M. V. K.) 
Ref. and Syn.—Geol. Russ. t. 13. f.'7. =O. virgata, Sow. Sil. Syst. t. 20. f. 15. 
Sp. Ch.—Suborbicular, apical angle about 145°; hinge-line slightly less than the width of the shell, greatest 
width about the middle of the length; receiving valye most convex, with a prominent beak, incurved nearly 
to the level of the lateral margins of the shell, greatest depth a little behind the middle ; cardinal area moderate, 
triangular, inclining backwards at an angle of 145°; entering valve uniformly convex, or with a slight longi- 
tudinal depression in the middle, about one-third the depth of the receiving valve; cardinal area very narrow, 
beak not projecting; surface of both valves radiated, usually with from eighteen to thirty-two simple, very 
regular, sharply defined, round or subquadrate ridges, the twelve or fourteen middle ones of which reach the 
beak, the lateral ones reaching the hinge-margin, the intervening spaces usually equal to the ridges in size 
and shape, and having one or three fine longitudinal striz towards the margin (never forming intermediate 
ribs) ; cast of the entering valve shewing the slit of a small rostral tooth in the triangular pad of the foramen ; 
two small pits of the cardinal teeth; impressions of the posterior pair of adductors, obtuse, prominent, separated 
by a deep mesial suleus, which becomes wider and shallower towards the front margin; casts of the receiving 
valve shew two short, thick, subparallel dental lamellze, and sharp narrow elevated ridges, extending nearly to the 
beak. Width (of small specimen) eight lines, proportional length of entering valve #4, width of cardinal area ¥%,, 
depth of both valves 34, (length occasionally one inch two lines). 
