216 BRITISH PALASOZOIC FOSSILS. [ Bracuiopopa. 
dollian, three miles from Ballintrae, Ayrshire; ditto Colmonel on the Stinchar; in the Bala limestone of 
Llandeilo, Caermarthenshire (finely-marked specimens) ; limestone of Trownscoed Gaerfawr. A fine-ridged 
variety in the limestone of Mulock Quarry, Dalquorhan, near Girvan, Ayrshire. 
OrtHIs crisPpA (M°Coy). Pl. 1 H. fig. 43. 
Ref.—M ‘Coy, Syn. Sil. Foss. Irel. t. 3. f. 10. 
Sp. Oh.—Truncato-elliptical, length exceeding half the width. depressed, hinge-line as wide as the shell, 
cardinal angles nearly rectangular ; cardinal area of receiving valve flattened, triangular, five times wider than 
high, and forming an angle of about 95° with the lateral margins; area of entering valve linear, almost 
obsolete, bent inwards nearly in the plane of the lateral margins; receiving valve gently convex ; entering valve 
nearly flat; surface of both valves covered with close, prominent, obtusely-angular, subequal ridges, six to eight 
in two lines at four lines from the beak, or forty-five to fifty at the margin, separated by slightly narrower 
spaces, and branching two or sometimes three times between the beak and margin, the ridges and interspaces 
crossed by regular, erect, squamose, concentric lines of growth, about the thickness of the ridges apart. 
Average width nine lines, proportional length varying from ,, to {;,, depth ;, height of cardinal area jj. 
I think it possible that the Orthis scabrosa (Davidson), figured in the Lond. Geol. Journ. Vol. I. t. 13, 
f. 14 and 15, may be the adult of this species, which is remarkable for the strong, regular, scale-like transverse 
seulpturing of its branching ridges. 
Position and Locality—Not uncommon in the schistose and calcareous beds of the Bala rock of Bala, 
Merionethshire; Bala limestone of Helms Gill, Dent, Kendal. 
Explanation of Figures —P1. 1. H. fig. 43. Natural size from the Upper Bala rock of Bala.—Fig. 43a. 
Profile of entering valve, and cardinal area of receiving valve-—Fig. 434. Surface magnified. 
ORTHIS ELEGANTULA (Dail.) 
Ref. and Syn.—Dalman, t. 2. f. 6. Davidson, Lond. Geol. Journ. t. 13. f. 10 (enlarged two diameters) 
= Orthis canalis Sow. Sil. Syst. t. 13. f. 12. and (Var. a. antiquior M°Coy) t. 20. f. 8. 
Sp. Ch.—Longitudinally obovate, hinge-line slightly less than the width of the shell, which is greatest at 
one-third the length from the beak of the entering valve, sides converging towards the front, which in old 
specimens is narrowed and produced in a small lobe; receiving valve very convex along the middle, from which 
the sides slope with slight convexity to the margins, greatest depth at one-third the length from the beak, 
which is very large and so much incurved as to reach the plane of the lateral margins ; cardinal area triangular, 
sharply defined, height usually one-fourth the width, but varying slightly in inclination and curvature, nearly in 
the plane of the lateral margins ; entering valve nearly flat, slightly convex on the sides, with a narrow mesial 
depression strongest near the beak, becoming gradually replaced, after two or three lines, by a slight mesial 
convexity, which gradually increases with age, so that this valve at three lines from the beak shews a slight 
wave in the front margin, convex towards the receiving valve; when about four lines the margin is horizontal, 
and when seven lines long, there is a slight wave, with the convexity towards the entering valve ; striz very 
obtusely angular, nearly equal at the margin (intervening sulci much narrower than the ribs), increasing in 
number by occasional dichotomy, twelve or. thirteen in two lines at three lines from the beak of entering valve ; 
about ten in same space at margin of large specimens (six lines long), on which they are strongly arched in a 
divaricating manner from the beak and mesial line, leaving an irregular triangular packet of two or four striz 
down the middle finer than the rest; striae more equal on the receiving valve, with a mesial parcel of three or 
five finer than the adjoining ones; interior of entering valve, with two very large, diverging, cardinal teeth, 
bordering two deep, ovate, cardinal pits, and a small, triangular, rostral tooth; four rounded, subequal, very 
faintly marked muscular impressions, the posterior pair deepest but rather smaller than the anterior, occupy 
the visceral space, bounded by very obtuse, doubly incurved, external ridges on each side, and an obscure, 
obtuse, longitudinal and transverse ridge, forming a crucial impression ; margin marked with radiating im- 
