220 BRITISH PALAZOZOIC FOSSILS. [Bracniopopa. 
sulci narrow, marked with very coarse punctures or little pits, and crossed by coarse, obtuse, transverse striz ; 
twelve to fourteen strie in two lines, at four lines from the beak; internal cast of receiving valve radiated 
with coarsely punctured impressions of the external striz ; cardinal teeth very short, thick, diverging at 80°. 
Width one inch, proportional length 7, depth ji. 
This is an extremely beautiful species, remarkable for its flatness and broad divaricating bands of coarse, 
branched, strive, which are not at all arched along the hinge-line, as in the somewhat similar 0. refrorsistria 
(in which the depth is greater, the striz much more uniform, the surface smoother, and the internal casts 
quite different). 
Position and Locality—Very abundant in the oolitie limestone and decomposing schists over the Bala 
limestone at Aber Hirnant, E. of Bala, N. Wales; and in the similar limestone of Cwm yr Aethen above 
Llanrhaider; sandy Bala schists of Maes Hir, N. E. of Aber Hirnant, N. Wales; and oolitic Bala lime- 
stone of Maes-y-fallen, Bala, M erionethshire ; rare in the Bala limestone of Cerrig-y-Druidion, Denbighshire. 
Explanation of Figures —PI. 1. H. fig 11. Natural size of rather small specimen, from the Bala lime- 
stone of Aber Hirnant; 11a, portion of surface of ditto magnified to shew the fasciculated striation ; 11 4, 
longitudinal section ; 11¢, portion of internal cast of receiving valve, shewing the hinge-teeth. 
ORTHIS HYBRIDA (Sov.) 
Ref.—Sow. Sil. Syst. t. 13. f. 11. 
Sp. Ch.—Rotundato-trigonal ; valves equal or nearly so; beaks very small; hinge-line very short, only half 
the width of the shell; greatest width near the front margin, which is slightly sinuate in the middle, but 
not raised into a wave; valves almost evenly convex, cuneiform, greatest depth at one-third the length from 
the beak, thence sloping towards the margins; entering valve with a very small mesial concavity; surface 
of both valves with a few thick varices of growth towards the margin; surface uniformly radiated with 
close, obtusely rounded stri of nearly equal size and closeness in all parts of the shell, increasing in 
number towards the margin, principally by interpolation, very rarely by dichotomy, fourteen in the space 
of two lines at the margin of a specimen three lines long: interior of entering valve with two very pro- 
minent, thick, slightly diverging cardinal teeth, and rather large rostral tooth; margins crenulated for a 
short distance by the external ribs; cardinal teeth bordering the open foramen in receiving valve very strong. 
Average width four lines, proportional length of receiving valve 4, length of entering valve slightly less, depth 
of both valves =. 
Position and Locality—Common in the Wenlock shale of Dudley, Staffordshire ; Wenlock limestone 
of Wenlock, Shropshire. 
ORTHIS LUNATA (So7.) 
Ref. and Syn. = Orthis lunata Sow. Sil. Syst. t. 5. f. 15. + O. orbicularis, t. 5. f. 16. 
Sp. Ch.--Suborbicular, or rotundato-quadrate: greatest width in front of the middle, the broad front 
margin being flattened or obscurely sinuate ; valves very much depressed ; receiving valve gently convex, most 
so at one-third from the beak; entering valve nearly similar, with a very faint, wide mesial depression towards 
the margin, not marked near the beak ; hinge-line nearly as wide as the shell ; cardinal area very small, inclined 
so as to bring the smali beaks nearly in contact; surface of both valves covered with very fine, close, obtuse 
strie, very frequently branching so as to remain almost equal in size on all parts of the shell, or from 
the sudden branching the ribs often closer and more numerous at the margin, where a specimen four lines 
long, has eighteen or nineteen in the space of two lines: casts of receiving valve shew two, much incurved, 
narrow slits of dental lamellze, bordering the rostral portion, extending two-thirds the length of the shell ; 
casts of entering valve shew two remarkably slender slits of cardinal teeth diverging at only 40°; muscular 
impressions remarkably elongate, oblong, extending rather more than half the length of the shell, sepa- 
rated by a broad mesial sulcus, left by a thick mesial ridge, and bordered externally by a narrow slightly 
