Bracuropopa. ] LOWER PAL/EOZOIC MOLLUSGA. 227 
ORTHIS SAGITTIFERA (M*Coy). Pl. 1. H. fig. 15, 16, 17, 18, 19. 
Ref—Ild. M°Coy, Ann. Nat. Hist. 2nd Series, Vol. VIII. p. 398. 
Sp. Ch.—Rotundato-quadrate, or oblong, length usually a little greater than the width, depressed ; hinge- 
line as wide as the shell; receiving valve obtusely subcarinate along the middle, most so near the beak, which 
is projecting, pointed, very slightly incurved ; apical angle 140°; profile only slightly curved; lateral margins 
horizontal, or with a slight mesial wave towards the receiving valve; cardinal area of receiving valve concave, 
triangular, four times wider than high, inclining backwards at 120°, triangular foramen narrow, open to the apex ; 
entering valve gibbous, deeper than the other, greatest depth a little behind the middle, profile strongly arched ; 
cardinal area very narrow (about one-third the height of the other, to which it is nearly at right angles, or in the 
plane of the margin ; beak small, depressed, with a deep narrow mesial sulcus, extending from it about half way 
to the margin: surface of both valves radiated with very close, numerous, obtuse, subequal, irregularly branching 
strize, separated by much narrower, deep, coarsely-punctured sulci ; about nine in two lines at six lines from the 
beak, straight in the middle and along the hinge-line, slightly arched divaricatingly, and most branched at the 
sides; no fasciculation: internal cast of receiving valve without mesial septum; two short dental jamellz 
diverging from the beak at 80°, forming part of the lateral boundaries of a short, strongly-defined, prominent, 
flattened, undivided triangular pair of muscular impressions slightly wider than long, scarcely one third the 
length of the shell, usually tripartite by a pair of wide longitudinal sulci; rest of surface finely radiated by 
impressions of the external striz; cast of entering valve with the triangular boss of the foramen slit by a 
very slender rostral tooth, from which a narrow, obtusely-angular furrow (corresponding with the external 
one) extends about half the length of the shell ; cardinal teeth forming long deep sulci, rather more than one- 
fourth the length of the shell, and diverging at 60°, resembling the mark of a broad arrow-head, of which the 
mesial sulcus was the shaft; surface impressed by the external radiating striee. Width (of small specimen) 
eight lines, proportional length of receiving valve [%, of entering valve ,;,, depth of receiving valve about =, 
depth of entering valve varying from = to 3. Length often upwards of one inch, the length and depth greater 
in proportion to the width in those old specimens, 
This species is, perhaps, most nearly allied to the O. twrgida (M°Coy), from which it is distinguished 
externally by its flatter receiving valve, with nearly straight profile, and a cardinal area less than half the height, 
and a much more obtuse apical angle ; the strize are thicker and coarser ; internally the difference is still greater, 
the present species having much shorter and wider muscular impressions in the receiving valve, and the 
characteristic broad-arrow-shaped impressions of the rostral portion of the entering valve, produced by the 
comparatively long, diverging, dental lamellie, wholly separated from the mesial furrow. The gibbous entering 
valve, with its deep, narrow, mesial sulcus near the beak, coarser strize, and totally different internal characters, 
separate it from the O. retrosistria (M°Coy), with which it also frequently occurs. 
Position and Locality—Extremely abundant in the decomposing sandy Bala schists of Aber Hirnant, 
E. of Bala, N. Wales. 
Explanation of Figwres—P\, 1. H. fig. 15. Natural size of rather small specimen of receiving valve, 
from the schists of Aber Hirnant.—Fig. 15a. Portion of surface magnified—Fig. 16. Rostral portion of 
entering valve, shewing the mesial sulcus, cardinal area, and foramen.—Fig. 17. Internal cast of entering 
valve.—Fig. 18. Longitudinal section of both valyes.—Fig. 19. Transverse section. 
ORTHIS SARMENTOSA (J/°Coy). Pl. 1. H. fig. 25, 26, 27, 28. 
Ref —MCoy, Silurian Foss. of Irel. t. 3. f. 17. 
Sp. Ch.—Transversely oblong, depressed ; hinge-line as wide as the shell, ears rectangular ; receiving valve 
elevated towards the beak, which is prominent, flattened towards the margin; cardinal area large, triangular, 
inclining backward at 120°, its height (in narrow specimens) one-fourth of the width; entering valve gently 
convex along the middle, depth greatest at one-third the length from the beak, slightly exceeding that of the 
Ga2 
