296 F. R. Cowper Reed—Fossils from Dufton Shales. 
rapidly attaining the size of the primary ribs; and being set at. the 
same distance apart they are indistinguishable at the margin from the 
primaries. 
Interior of pedicle-valve with short triangular hinge-teeth and 
small sub-ovate muscle-scar, about one-fourth the length of the valve, 
rather deeply sunk and circumscribed by a ridge, weakly bilobed in 
front and composed of two small diductors separated by very narrow 
linear adductors. Pair of median vascular sinuses, contiguous and 
parallel at first, run forwards from muscle-scar, ultimately diverging 
and curving outwards laterally. 
Interior of brachial valve with narrow straight linear cardinal 
process borne on rather massive thick hinge-plate, continued anteriorly 
as broad low rounded ridge between deeply sunk but indistinctly 
defined posterior adductors; anterior adductors not visible; dental 
sockets deep, with strong-walls and stout prominent triangular crura. 
Average Dimensions.—Length 30mm.; width 25mm. The smaller 
(younger) examples are usually somewhat broader. 
Remarks.—In shape this shell much resembles some examples of 
O. calligramma, Dalm., but the species O. plicata, Sow.,' which Davidson 
regarded as merely a variety of it, is closely similar, especially in 
internal characters, but the cardinal angles are not really pointed or 
mucronate in O. duftonensis (though internal casts often give an 
erroneous impression), nor are the ribs angular, nor the interspaces of 
less width than the ribs. The typical O. calligramma is more convex, 
the beak of the pedicle-valve more incurved, and the ribs fewer, but its 
general shape and internal characters are very similar, and it agrees 
also in the simplicity of the ribs and ornamentation. The form 
ascribed to this species by Wiman,*? from the Asaphus Limestone of 
the Baltic province, appears to bear a considerable resemblance to our 
species. 
So much confusion and indefiniteness has been caused by putting 
well-marked varieties (or species) occurring on different stratigraphical 
horizons into O. calligramma, that it seems desirable to separate 
specifically this very strongly characterized local form under the name 
duftonensis. : 
ORTHIS MELMERBIENSIS, sp. nov. Pl. XXIII, Figs. 4-8. 
Shell sub-quadrate, strongly folded along median line with dependent 
lateral lobes; anterior margin angulated strongly in middle; hinge- 
line straight, equal to or slightly greater than width of shell; not 
auriculate. Pedicle-valve swollen, convex, angulated longitudinally 
down middle, sub-carinate, with lateral portions hanging down and 
flattened; beak small, elevated, prominent, rising above hinge-line, 
slightly incurved; hinge-area triangular, rather large, vertical or 
steeply inclined. Brachial valve divided into two lobes by strong 
deep median sulcus, rapidly increasing in depth and width towards 
front margin, which is angulated; beak small, with narrow hinge- 
area. Exterior of shell ornamented with about 30 fine, closely set 
é 1 Davidson, Brit. Foss. Brach., vol. iii, p. 245, pl. xxxv, figs. 25, 26; pl. xxxvii, 
ae al 
m2 Wiman, Bull. Geol. Inst. Upsala, 1907, vol. viii, p. 103, pl. vii, figs. 28-30. 
