282 BRITISH PALA®OZOIC FOSSILS. [ LamMELLIBRANCHIATA. 
CARDIOLA FIBROSA (Sow.) 
Ref—ld. Sow. Sil. Syst. t. 8. f. 4. 
Sp. Ch.—Obliquely ovato-rhomboidal, convex, most so about one-third from the beaks, which are large, 
prominent, obtuse; anterior side very short, scarcely extending as far forward as the beaks, ventral margin 
gently convex, posterior end very oblique, elliptical, the respiratory angle rounded ; hinge-line very short, and 
anal angle very obtuse ; surface with a few deep, concentric wrinkles, about one line apart at six or eight lines 
from the beak; numerous fine, sharp, concentric strize crossed by equally fine, close, thread-like, rounded ridges 
from the beak, about five in the space of one line in the middle of the shell. Length from anterior to posterior 
end ten lines, proportional width at right angles to middle of ventral margin =, depth of one valve 
The fineness of the longitudinal and smaller concentric suleation, as well as the more oblique form, easily 
separate this species from the more common C. interrupta. The two nearly equal, ovate, adductor impressions 
are often plainly seen in casts. Both the longitudinal and concentric markings extend quite to the beak, and 
must have been accidentally absent in Mr Sowerby’s examples. 
Position and Locality—In the Lower Ludlow rock above Park-lane, Llandeilo. 
20 
100° 
JARDIOLA INTERRUPTA (Brod.) 
Ref. and Syn.—ld. Broderip, Geol. Proc. 1844; Sil. Syst. t. 8. f. 5 = Cardium cornu-copicee Goldf. Pet. Germ. 
t. 143. f. 1. (1848). 
Sp. Oh.—Ovato trigonal, slightly oblique, gently convex ; beaks obtuse, prominent ; ventral and posterior 
margins forming one semicircular or slightly elliptical curve ; surface regularly radiated with subequal, close, 
obtuse ridges (four in two lines in the middle of the shell), slightly smaller at the sides, interrupted by deep, 
irregular, concentric sulci from one to two lines apart. Length one inch one and half lines, proportional 
width at right angles to middle ,{;, depth of one valve ze 
Position and Locality—Abundant in the green mudstone of the Lower Ludlow of Sugar-loaf, and at Green 
Quarry, Leintwardine, Shropshire; Yr alt, Welchpool; Cwm Craig Ddu, Builth, Brecknockshire ; Erw Gill 
Fach, Builth; Llangynyw Rectory, Montgomeryshire; Troutbeck, Applethwaite, Westmoreland ; and in the 
black schists of Cefn-ddu; hills W. of Penny Bridge, N. Lancashire; Builth Bridge; Mynidd y gaer, S. side, 
Llanefydd, near Ruthin, Denbighshire; Maen Goran, Llangollen, Denbighshire ; Coniston flags, Helms Knot, 
Dent, Kendal, Westmoreland. 
3rd Family. ARCIDA. 
Shells equivalve, or subinequivalve, orbicular or oblong, quadrate, often with a thick-piled epidermis; a 
triangular, external, ligamentary facet under each beak, occupied by the ligament, which is external; hinge of 
very numerous teeth on a straight or arched line; two muscular impressions in each valve: animal large, 
mantle entirely open ; foot moderate, sometimes tendinous ; gills filamentous. 
Distinguished from the Nuculide by the ligamentary facet beneath the beaks. 
Genera :—Arca (including Byssoarca and Cucullea as subgenera), Pectwnculus ? 
Genus. ARCA (Linn.) 
Gen. Char.—Shell elongate, oblong, angulated, equivalve or slightly inequivalve, margin closed all round ; 
one triangular, anterior, muscular impression, and one lengthened posterior one ; ligament external, linear, on 
the inner edge of the ligamentary facet, which is striated and marked with angular sulci; hinge-line straight, 
with very numerous transverse teeth; animal elongate, foot often truncate and bearing horny fibres instead of 
byssus ; gills in long bands of free filaments. 
There are several subgenera in Arca, as above defined, distinguished from Pectunculus by their elongate 
form and straight hinge-line. The subgenus Byssoarca has a sinus in the middle of the ventral margin for 
the passage of a large byssus. 
