Bracuiopopa. | LOWER PALAZOZOIC MOLLUSCA. 219 
105°); beak slightly projecting, not incurved: entering valve very convex, evenly tumid, depth greatest half 
way between the beak and margin, varying in proportion ; beak small, incurved quite to the level of the margin 
of the valve; cardinal area very narrow, bent inwards horizontally, parallel with the plane of the lateral 
margins, and at right angles to large area of receiving valve; surface of both valves radiated with about 
twenty to thirty (most usually about twenty-four) obtusely angular or rounded regular simple ribs, all extending 
to the beak, separated by spaces equalling them in size and shape, in which are usually developed towards 
the margin one or two short intermediate ribs (not striz, as in O. ca/ligramma,) unequally developed, often 
appearing between some only of the ribs on a given specimen; substance of the shell thick, coarsely fibrous, 
the ends of the fibres, obliquely puncturing the surface, which is marked with very minute transverse lines of 
growth, but never having longitudinal striz; casts of the flat receiving valve with two strong dental lamellz 
diverging at an angle of 85°, enclosing a very prominent, subquadrate pair of muscular impressions, reaching 
nearly half way from the beak to the margin, bilobed by an undefined depression only at the posterior end ; 
margin of the valves most usually with a border two or three lines wide of abruptly prominent casts of the ribs 
(often bifid), from which in a small number of specimens faint extensions or obsolete ribs approach the beak ; 
casts of convex entering valve shew two short diverging pits for the thick cardinal teeth close to the depressed 
beak, with a mesial triangular pit produced by the rostral tooth dividing the triangular pad of the foramen 
in the small horizontal area ; greatly elevated and a little in front of the cardinal teeth are the two obscurely 
rounded bosses from the posterior pair of muscles, deeply separated throughout by a wide defined sulcus (not 
affecting the rest of the valve); anterior pair of muscles seldom perceptible, close in front of the large pair; 
surface nearly smooth, or striated with slight extensions of the ribs, which are abruptly prominent for a width 
of two lines round the margin, and often apparently very numerous, from the intercalated ribs being more 
strongly marked than on the exterior. Width one inch five lines, proportional length 7 to 7%, depth =; to 5, 
(sometimes less) length of area on convex entering valve =, on flat receiving valve {;,. 
This species varies in the depth of the convex entering valve and slightly in the proportion of width to 
length. Mr Salter, (Mem. Geol. Surv. Vol. II.) has well pointed out that the entering or most convex valve of 
this species corresponds with the flattened valve of the O. calligramma, and its varieties. The width of the 
ribs as well as the size, presence or absence of the intermediary short ribs, varies much in different parts of 
one specimen. 
Position and Locality —Extremely common in the Bala sandstone of Bodean, Caernarvonshire ; in 
ditto, Alt yr Anker, Meifod, Montgomeryshire; Coniston (or Bala) limestone of Coniston, N. Lancashire ; 
depressed var. in Bala schists of Llyn Ogwen, N. Wales; Bala schists of Gelli Grin, Bala, Merionethshire ; 
ditto of Llanwddyn, E. of Berwyn Mountains; in ditto S. of Cwm Lanerch, Bettws, Caernarvonshire ; 
calcareous Bala flags, hill N. of Moel Uchlas, Montgomeryshire ; Bala schists of Cader Dinmael, near 
Corwen, Denbighshire ; in ditto of Cefn y Coed, Llangedwyn, Montgomeryshire; Bala schists, Glyn Cei- 
riog, S. of Llangollen, Denbighshire; Bala schists of Llansantfraid, Glyn Ceiriog, Denbighshire ; Lower Bala 
schists of Moel y garned, Uchaf, W. of Bala Lake, Merionethshire ; Bala limestone of Applethwaite Common, 
Westmoreland. 
Ortuts Hrrnantensis (M/°Coy). Pl. 1. H. fig. 11. 
Ref.—Id. M°Coy, Ann. Nat. Hist. 2nd Series, Vol. VIII. p. 395. 
Sp. Ch.—Truncato-orbicular, depressed; cardinal area very low, triangular, nearly twelve times wider 
than high; hinge-line slightly less than the width of the shell; cardinal angles slightly obtuse; lateral and 
front margins horizontal, almost uniformly curved; entering valve perfectly flat, with a slight longitudinal 
mesial depression near the beak; receiving valve slightly and gently convex, most so along the middle at 
about one-third the length from the beak: both valves with numerous, slightly unequal prominent, angular, 
strongly fasciculated strive; each of the primary ridges branching near the middle into from five to seven 
smaller, forming in some specimens slightly angulated, divaricatingly arched groups; eight or ten strive at 
the cardinal angles, smaller and straighter than the rest, running nearly parallel with the hinge-line ; separating 
FF2 
