226 : BRITISH PALAOZOIC FOSSILS. [ Bracutopopa, 
margins, which are marked by a narrow border of the external strie. Width eight lines, proportional 
length of receiving valve =, of entering valve 7),, depth about =. 
The striation of this species is minutely granular under the lens, as in the O. elegantula, with which 
imperfect specimens might very readily be confounded ; but the strize are coarser, much more equal, obtuse 
and close together, in that species, dichotomise, and do not present the subalternate character of the finer 
striation of this species, which is narrower, and very easily distinguished by the broad, flat, mesial depression 
in the receiving valve, when that part is seen. Some of the specimens shew the hinge-line more extended, 
and the mesial ridge in the entering valve defined, leading to the belief that the O. fallax (Salt.) may be 
an extreme variety of the present species, mainly distinguished by a coarser striation. 
Position and Locality—Extremely abundant in the greenish sandstone of Mulock Quarry, Dalquorhan, 
near Girvan, Ayrshire. 
‘Ortuis RIGIDA (Davidson). 
Ref.—Davidson, Lond. Geol. Journ, t. 13. f. 16 to 17. 
Sp. Ch.—Rotundato-quadrate ; cardinal angles obtuse; hinge-line slightly less than the width of the 
shell, which is greatest about the middle; receiving valve evenly convex; greatest depth about the middle, 
with a very high, flat, triangular area, its height nearly one-third the width, inclined backwards at 95°; 
triancular foramen long, narrow, open; entering valve slightly convex, but the depth less than one-half of 
that of the receiving valve; cardinal area very narrow (one-third the height of that in the receiving valve), 
inclining inwards at right angles, or nearly so, to the plane of the cardinal area of the receiving valve ; both 
valves radiated with about fifty prominent, narrow, obtusely angular ridges, the primary ones usually branching 
at two or three lines from the beak, separated by equal and similar sulci, having sometimes three or four 
minute longitudinal stria in each sulcus under the Jens, the whole being crossed by close, sharp, transverse 
strie. Width one inch, proportional length of receiving valve ;;, proportional length of entering valve =, 
depth of both valves =. 
This species is totally distinct from the Orthis calligramma and O. plicata, with which Mr Salter proposes 
to unite it (Mem. Geol. Surv. Vol. II. p. 376), by its extremely large triangular cardinal area in the receiving 
valve placed at nearly right angles to the plane of the margins. 
Position and Locality.—Not uncommon in the schists on the Bala limestone at Bryn Melyn Quarry, near 
Bala, Merionethshire; ?sandy Bala schists of Alt yr Anker, Meifod, Montgomeryshire ; sandy Bala schists of 
Gaer Fawr, Welchpool, Montgomeryshire. 
OrTHIS RUSTICA (Sow.) 
Ref.—Sow. Sil. Syst. t. 12. f. 9. 
Sp. Ch.—Rotundato-quadrate, depressed ; hinge-line equalling the width of the shell; sides subparallel, very 
slightly convex; front slightly convex ; profile of receiving valve nearly straight, greatest height at the beak, from 
which the surface slopes gradually to the margins ; cardinal area triangular, four times wider than high, inclined 
backwards at an angle of 103°; entering valve nearly flat, gently convex on the sides, obscurely channeled on 
the middle near the beak; surface of both valves radiated with thick subangular ridges, equal in thickness, 
and about fifty in number near the margin of a specimen nine lines long, simple, but of unequal lengths, 
separated by deep concave spaces equalling them in width (six to seven ridges in a space of three lines, at nine 
lines from the beak); both ridges and sulci crossed by strong, regular, close, suberect laminze or lines of growth. 
Width one inch two lines, proportional length of receiving valve {j;, of entering valve ;{,, depth of entering 
valve ;4, depth of receiving valve =. 
I agree with Mr Davidson (Lond. Geol. Journ. p. 64) and Sowerby in thinking this species completely 
distinct from those in the Caradoc sandstone and Bala rocks Mr Sowerby’s figure represents the spaces 
between the ribs, and he omits the strong transverse sculpturing, which however is shewn in Mr Davidson’s 
figure, in the work above quoted. 
Position and Locality—Not uncommon in the Wenlock limestone of Wenlock, Shropshire. 
