Bracaropopa. | LOWER PALAXOZOIC MOLLUSGA. 223 
OrtHIs porcAaTA (MCoy). Pl. 1. H. fig. 41 and 42. 
Ref. and Syn.—M Coy, Sil. Foss. Irel. p. 32. t. 3. f. 14 (1846) = O. inflata Salt. Mem. Geol. Surv. 
Vol. Il. p. 372. 
Sp. Ch.—Transyersely oblong, rounded ; receiving valve flat, very slightly prominent towards the beak, 
which is small and not incurved, becoming slightly depressed in the middle of old shells, by a very wide, shallow, 
concavity, scarcely affecting the margin; cardinal area varying in height, usually about eight times wider than 
high, flat, triangular, inclining back at 115°; foramen acute, open to the point of the beak : entering valve of very 
great but variable gibbosity; sides tumid, much arched from the very small beak to the front margin, which 
is raised by a scarcely perceptible, wide sinus, the greatest depth being a little behind the middle; cardinal 
area about one-third the size of that in the receiving valve, and lying nearly in the plane of the lateral 
margins ; both valves radiated with coarse, angular, subequal ridges, separated by shallow angular sulci (about 
sixty at nine lines from the beak), straight and nearly simple on the sides, but towards the middle some 
bifurcating once or twice, some dividing at once into three or four, and a few remaining simple, about five in 
two lines, at six lines from the beak, but varying in this respect; all the ridges crossed by fine, close strize 
of growth ; internal cast of entering valve shews the large triangular boss of the foramen, nearly occupied by the 
very large ovate pit of the rostral tooth; diverging on each side of which is a very broad pit of the short, thick 
cardinal tooth: posterior pair of muscular impressions large, ovate, slightly convex, marked with longitudinal 
or outward curved striz, separated by a wide rounded furrow of a mesial ridge, extending their length from the 
beak, and bounded externally by the faint depression of a slightly prominent margin ; anterior pair less than half 
the size of the posterior, obscurely defined into four or five radiating lobes faintly defined all round ; internal 
cast of receiving valve, with two thick dental short lamelle, diverging at 140°; muscular impressions flat, 
quadrate, wider than long, faintly defined by the sulcus of a slightly raised margin (formed by an extension 
of the dental lamellze) with nearly parallel sides, extending less than half the length of the shell, forming 
narrow lobes at the anterior lateral angles, projecting considerably in front of the nearly straight anterior 
boundary ; interior of both valves marked nearly from the beaks with narrow branching impressions of the 
external striz, strongest at the margin; substance of the shell densely fibrous. Width one inch five lines, 
proportional length usually 7 (rarely varying from “ to 4), depth of both valves varying from & to “. 
I originally described and figured this species from the Coniston limestone of Portrane, near Dublin, 
from a great abundance of specimens, which however from the nature of the limestone could only be extracted 
in fragmentary condition, still the characters were plain enough to distinguish the species from all then known. 
Upwards of a year afterwards Mr Hall, in his Paleontology of New York, carefully figured and described, 
under the names O. occidentalis and O. sinuata, a species equally common in the limestone (Trenton) of the 
same age in America, a representative closely-allied form, distinguishable probably from ours by the great 
comparative depth of the receiving valve, and its deep mesial sinus, which abruptly indents the margin. I do 
not think it desirable therefore to suppress his species, as would be necessary, if, as Mr Salter suspects, they be 
identical with the present species, which he inadvertently redescribes in the Mem. Geol. Surv. under the name 
O. inflata. The principal forking of the ribs takes place at from two to four lines from the beak, and again 
near the margin about seven lines from the beak of the entering valve. The substance of the shell is thick. 
By a peculiarity of weathering several of the specimens in the sandstone of Meifod shew a border of large, 
lacunose, pitting between the ribs, which are there extremely prominent: this appearance results from the 
imperfect removal of the substance of the shell; other specimens from the same mass of stone shewing the 
ordinary conditions as above described. 
Position and Locality—Very common in the Bala schists of Llansantfraid, Glyn Ceiriog, Denbighshire ; 
Bala schists of High Haume, Dalton in Furness; Bala schists of Cyrn y Brain, W. of Wrexham, Denbigh- 
shire; Bala schists of Blain y Cwm, W. of Nantyre, Glyn Ceiriog, S. of Llangollen, Denbighshire ; very 
abundant in the Bala limestone of Coniston, N. Lancashire; Caradoc limestone of Horderly S.; very common 
in the Bala sandstone of Alt yr Anker, Meifod, Montgomeryshire; var. in Bala schists of Pen y Park, Llan- 
fyllin, Montgomeryshire ; calcareous Bala flags of Corwen, Merionethshire; Bala sandstone of Alt y Gader, 
near Llanfyllin, Montgomeryshire. 
