F. R. Couper Reed—Fossils from Dufton Shales. 297 
subangular ribs, nearly straight, equidistant on margin and of equal 
size, mostly dividing at one-fourth to two-thirds their length and each 
bearing a single row of rather distant short tubular erect spines, 
specially developed on the lateral portions of the valves; a fine close 
concentric lineation is also present. Interior of pedicle-valve with short 
impressed sub-triangular muscular sear widening anteriorly and broadly 
excavated in front, about one-third the length and width of the valve 
and composed of a pair of divergent diductors enclosing elongated oval 
adductors; occasionally two straight narrow divergent vascular ridges 
are traceable running forwards from the anterior outer angles of the 
muscular scar to a concentric sub-marginal ridge (often present). 
Interior of brachial valve with small rounded knob-like cardinal 
process; no thickened hinge-plate; crura short, sub-triangular; low 
narrow median ridge running forwards for about one-half the length 
of valve from cardinal process; muscle-scars indistinct. 
mm, 
Dimensions. Length ‘ > 75 
Width : : 9-0 
Remarks.—This shell somewhat resembles O. vespertilio,! Sow.; but 
the ornamentation is different and the internal characters are’ not 
quite identical, the cardinal process is not raised on a hinge-plate nor 
the adductor muscles deeply sunk, while in the pedicle-valve the 
muscle-scar is not saucer-shaped, but triangular and sub-bilobate; the 
submarginal ridge and hollow spines also are special features. It 
might be thought that this shell is merely a variety of the protean 
O. testudinarva, Dalm., using this specific name in the loose and 
comprehensive manner customary. But it seems undesirable to stretch 
this species yet more; and indeed the internal characters and surface- 
spines are quite sufficient to separate the Dufton form. 
Orruis (ScrnipIum?) EQuivocaLis, sp. nov. Pl. XXIV, Figs. 1-4. 
Shell transversely sub-fusiform, widest along hinge-line, with 
acutely angular cardinal extremities. Pedicle-valve strongly convex, 
most so in middle pertion, cardinal angles somewhat flattened; beak 
high, incurved, with triangular, steeply inclined hinge-area ; muscular 
area about one-third length of valve, sub-quadrate, not deeply impressed, 
divided into two pairs of sub-parallel scars, the outer ones being the 
diductors; teeth short, strong; small apical foramen apparently 
present. Surface of pedicle-valve furnished with pair of equal 
median ribs, smaller than the rest, and rather more closely placed, 
with 7-8 stronger subangular ribs on each side, very slightly curved 
and somewhat decreasing in strength towards lateral angles. Brachial 
valve moderately convex, but much less so than opposite valve; 
bilobed, with shallow median sinus widening anteriorly to over one- 
third the width of the valve; small, inconspicuous beak, and narrow 
hinge-area; narrow linear septum present, about one-fourth the 
length of the valve, with sub-parallel crural plates of same length on 
each side ; muscular impressions indistinet. Surface of brachial valve 
with single straight median rib rather smaller than the rest, flanked 
1 Davidson, Brit. Foss. Brach., vol. iii, p. 236, pl. xxx, figs. 11-21. 
