LAMELLIBRANCHIATA. ] LOWER PAL/ZOZOIC MOLLUSCA. 283 
Arca Epmonpiirormis (M‘Coy). Pl. 1. K. figs. 2 and 3. 
Ref.—Id. M°Coy, Ann. Nat. Hist. 2nd Series, Vol. VII. p. 52. 
Sp. Ch—Oblong, obtusely subquadrate, very gibbous; beaks very large, tumid, about one-fourth of the 
length from the anterior end, which is obtusely rounded; ventral margin slightly sinuate in the middle ;_poste- 
rior end very slightly oblique, obtusely rounded ; hinge-line nearly straight, as long as the shell, with numerous 
minute teeth inclining slightly towards the beak; surface smooth or with minute wrinkles of growth. Length 
seven and half lines, proportional width =; greatest depth of one valve (at middle) =. 
This species resembles a small Modiolopsis or Edmondia in form, but in some of the specimens the 
hinge-teeth are seen as in Arca, except that they incline slightly towards the beak instead of from it. 
Position and Locality.—In the fine sandy beds near Llangynyw Rectory, near Welchpool, Montgomery- 
shire; sandstone of Alt y Gader, near Llanfyllin, Montgomeryshire; in the Upper Ludlow rock of Benson 
Knot, Kendal, Westmoreland; Moel Seisiog, Llanrwst, Denbighshire; Coed y Bedw, Bala, Merionethshire ; 
Aber Hirnant, E. of Bala, N. Wales. 
Explanation of Figures.—P1. 1. K. fig. 2, natural size, right valve; fig. 3, left valve. 
ARCA PRIMITIVA (Phill.) 
Ref.—ld. Phillips, Mem. Geol. Surv. Vol. II. Pt. 1. t. 21. f. 5. 
Sp. Ch.—Subtrigonal, elongate ; beaks large, obtuse, very prominent, nearer to the posterior than the 
anterior end; valves gibbous, most so towards the beaks, from which an angular ridge extends on each side 
towards the inferior, anterior, and posterior angles, defining at each end a steeply inclined slope ; anterior end 
long, narrow, obtusely subtruncate, rounded at the end; posterior end wide, obliquely subtruncate ; hinge-teeth 
large, very slightly diverging from the beak; muscular impressions strong; surface with numerous concentric, 
step-like ridges of growth, about half a line wide, crossed by very numerous, close, fine, radiating striz ; length 
five and half lines ; in proportion to length, greatest width (from beak to opposite margin) ;;, from beak to 
anterior end *, width of anterior end =, width of posterior end =, greatest depth of one valve =. 
Position and Locality—Rare in the Upper Ludlow rock of Benson Knot, Kendal, Westmoreland, and 
High Thorns, Underbarrow, Kendal, Westmoreland. 
Arca (Byssoarca) SUBAQUALIS (M*°Coy). Pl. 1. K. fig. 1. 
Ref —ld. M°Coy, Ann, Nat. Hist. 2nd Series, Vol. VII. p. 51. 
Sp. Ch.—(Cast) Oblong, equivalve, nearly equilateral, ends subtruncate, rounded, very gibbous in the 
middle, about twice as long as wide; depth of both valves equal to the width; beaks very large, obtuse, tumid, 
marked on the sides with four or five large wrinkles; a shallow sinus for the byssus in the ventral margin a 
little nearer the anterior than posterior end, and slightly obliterating the simple pallial scar ; adductor impressions 
deep, rounded ; hinge-teeth very numerous, small, equal, at right angles to the hinge-line. Length one inch 
eight lines; proportional width =,. 
Mr Sowerby seems to have united two different fossils (the differences between which have been noticed 
by various writers) under the one name Arca Hastnori (see t. 20. f. 1 of the “Silurian System”) : the fig. 1 a 
from Eastnor Park should, from the name, be considered the type of the species, and is a regular, subeom- 
pressed, oval shell, without ventral sinus; while the other, fig. 14, of which Mr Sowerby says, “If it be not an old 
shell grown very thick, it may be a different species,” may, I think, possibly be referred to the present species. 
Position and Locality—In the grey micaceous sandstone of Llechclawdd, near Llandovery, S. Wales. 
Explanation of Figures —P1. 1. K. fig. 1, natural size, seen along the hinge-line to shew the numerous 
equal hinge-teeth ; fig. 1a, same specimen, side view. 
Genus. CUCULLELLA (J/°Coy). 
Ref.—Ann. Nat. Hist. 2nd Series, Vol. VII. p. 50. 
Gen. Char.—Subrhomboidal, inequilateral, subequivalve; margin even; hinge-line entirely crenulated ; 
muscular impressions two, with a simple pallial scar between them; a strong internal septum extends from 
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