224 BRITISH PALAOZOIC FOSSILS. | Bracuioropa. 
ORTHIS PROTENSA (So7.) 
Ref.—Sow. Sil. Syst. t. 22. f. 8 and 9. 
Sp. Ch.—Truncato-elliptical , cardinal area low, triangular in each valve; hinge-line equalling or slightly 
exceeding the width of the shell ; depressed, varying from longitudinal to transverse ; receiving valve nearly flat, 
with a slightly projecting flattened beak, from which a broad angular ridge extends to the margin; entering 
valve rather deeper in most specimens than the receiving one, gently arched, greatest depth about half way 
between the beak and margin ; a deep longitudinal depression extends from the beak to the margin ; both valves 
radiated with very numerous, obtusely-angular, fasciculated ridges, separated by narrower angular, sulci, branching 
twice or three times at irregular distances from the beak to the margin, where each of the original ones forms 
four or five more slender, about fourteen (eleven to fifteen) in two lines at four lines from the beak, about six 
in one line at two lines from the beak, slightly arched on the sides, nearly or quite obsolete on the ears, 
which are marked by a few irregular concentric plicee of growth; internal cast of receiving valve with two very 
short cardinal teeth diverging at 75°, slightly incurved at their extremities, which are connected by a transverse 
furrow completing the boundary of a trapezoidal muscular impression, less than one-fourth the length of the 
shell; the internal cast of entering valve with the narrow triangular boss of the foramen divided by a slender 
rostral tooth; cardinal teeth very short, parallel with the hinge-line; substance of the shell moderately thick, 
densely fibrous. Width seven lines, proportional length ;%, depth =. 
The above proportional measurements seem to indicate the true form, although, as mentioned, specimens 
frequently occur in which the length equals or slightly exceeds the width. 
Position and Locality—Very common in the Bala schists of Goleugoed, Llandovery, Mandinan, Caer- 
marthenshire; Bala schists of Cader Dinmael, near Corwen, Denbighshire; very common (sometimes an inch 
wide) in the Bala limestone of Ash Gull, W estmoreland ; Bala schists of Mathyrafal, S. of Meifod, Mont- 
gomeryshire ; a blunt-angled wide variety in the olive shale of Llangynyw Rectory, near Welchpool, Mont- 
gomeryshire. 
ORTHIS RETRORSISTRIA (A/*Coy). Pl. 1. H. fig. 12. 
Ref.—Id. M*Coy, Ann, Nat. Hist. 2nd Series, Vol. VIII. p. 396. 
Sp. Ch.—Rotundato-quadrate, depressed, no mesial ridge or furrow in either valve; hinge-line nearly 
or quite as wide as the shell; cardinal angles slightly obtuse ; cardinal area flat, triangular, six or seven 
times wider than high in the receiving valve, and inclined backwards at about 120°; only one-third this 
height in the entering valve; rostral tooth of entering valve very large; triangular foramen of receiving 
valve, with an internal, semiconical, hood-shaped extension of the dental lamellze within it; receiving valve 
gently convex ; greatest depth about the middle of the length; entering valve flat round the margin, gently 
coneaye in the middle; both valves with a few concentric wrinkles of growth about a line apart, and 
radiated with slightly irregular obtuse striae, which branch into two or three at two or three intervals 
between the beak and margin, so that each of the strong primary ones forms from seven to ten at the 
margin, separated by a rather deeper sulcus from the adjoining ones, so as to produce a flat, indistinctly 
marked fasciculation ; intervening sulci about the same size as the striz, (obscurely punctured in some 
specimens), which are straight in the middle, gradually assuming a divaricating curve on the sides, which 
is so great near the angles, that a large number of the lateral strie curve backward from the beaks to 
terminate along the distal half of each side of the hinge-line instead of at the lateral margin; all the striae 
erossed by indistinct transverse lines of growth; the size of the strize does not vary much, in the various parts of 
the shell from fourteen to seventeen may be counted in two lines at four lines from the beak; cast of entering 
valve nearly smooth, except at the margin, which in the middle is marked with close slightly dichotomous lines ; 
the narrow triangular boss of the foramen is cleft by a deep, narrow, elliptical pit of the rostral tooth, flanked 
by the pits of two short cardinal teeth, diverging at 70°, between which and the hinge-line are the smaller but 
more diverging bosses left by the cardinal pits ; a deep straight sulcus extends from the rostral tooth nearly to 
the anterior margin, marking a very long obtuse mesial ridge, on each side of the rostral part of which is a sub- 
