Bracuiopopa. | LOWER PALALOZOIC MOLLUSGA. 229 
impressions of the beak, less than one-third the length of the shell; entering valve with two very short pits 
for cardinal teeth. Width nine lines, proportional length +, length of entering valve 3, depth 2. 
The slight convexity of the receiving valve, the wide and very low cardinal area, and the very much 
coarser, more separated, angular, and less equal ridging of the valves, easily separate the species from the 
O. elegantula, and the greater size and depression of the valves, as well as the less number of strix at 
small distance from the beak, separate it from the O. parva. Gothland specimens agree in general form, 
proportional measurements, and in the remarkably low cardinal area, but seem to have the entering valve 
often almost entirely concave, the receiving valve more obtuse, and the ribbing slightly more regular, but 
the observed varieties in the Gothland specimens shew these to be inconstant, and irregular peculiarities 
of a few specimens. I have not observed any remarkable transverse intercostal striz spoken of by some 
writers, either in our British or Gothland specimens, both shew the usual transverse continuous lines of 
growth at the margin, as in all shells, but in some states of preservation the sides of the ribs are 
crenulated by imbricating exfoliations, sometimes very strongly marked in partial casts. 
Position and Locality—Very common in the Caradoc limestone and fine sandstone of H orderly ; common 
in the limestone of the Hollies, Church Stretton, Shropshire; Bala limestone and sandstone of Alt yr Anker, 
Meifod, Montgomeryshire; very common in the partially oolitic Bala limestone of Maes-y-fallen, Bala, 
Merionethshire; fine Bala sandstone of Bodean, Pwllheli, Caernarvonshire; calcareous Bala schists of Dolydd 
Ceiriog, S.E. of Moel Ferna, E. of the Berwyn Mountains; Bala limestone of Fridd Cowny, Llanwddyn ; 
? Bala schists of Bala, Merionethshire; common in the Bala limestone of Mathyrafal, S. of Meifod, Mont- 
gomeryshire ; {Bala flags of Bryn Eithen, Penmachno, N. Wales (var. a with rather more numerous stria) ; 
Bala schists of Cyrn y Brain, W. of Wrexham, Denbighshire; var. a, Bala schists of Selattyn Road, S. of 
Llangollen, N. Wales; rare in the Bala schists of Moel Uchlas, Montgomeryshire ; Bala schists of Cader 
Dinmael (var.) near Corwen, Denbighshire; Bala schists of Pen y Park, Llanfyllin; Bala schists of Cwm 
Lanerch Bettws (S. of), Caernarvonshire; sandy Bala schists of Gaer Fawr, Montgomeryshire; rare in 
the sandstone of Dalquorhan, Ayrshire. 
ORTHIS TURGIDA (M°Coy). Pl. 1. H. fig. 20, 21, 22, 23, 24. 
Ref —Id. M°Coy, Annals Nat. Hist. 2nd Series, Vol. VIII. p. 399. 
Sp. Ch.—Longitudinally ovate, globose, margin very obtuse, greatest width slightly in front of the hinge- 
line, which is slightly less than the width of the shell; receiving valve very gibbous, profile much arched by 
the large beak declining nearly to the level of the lateral margins; greatest depth about the middle of the 
length; in old specimens there is a faint, wide, shallow depression towards the front margin, which gradually 
changes into an obscure prominence towards the beak or in the young; sides tumid; beak long; apical angle 
100°; entering valve extremely gibbous, sometimes hemispherical, in some specimes more depressed but re- 
maining remarkably tumid towards the margins; a deep narrow sulcus extends from the beak about. half 
way to the margin, becoming rapidly effaced by widening and flattening towards the front, which is depressed 
with a narrow sinus in young specimens (from the mesial sulcus), and very slightly raised with a wide shallow 
wave (not affecting the surface) in old individuals (from the slight mesial depression of the old receiving 
valve) ; both valves covered with numerous fine, sharply defined, obtuse striae, separated by flattened spaces 
equalling them in width, bifureating two or three times between the beak and margin, where in old individuals 
they are a little finer than in the middle of the shell, eight to ten in two lines at six lines from the beak, 
about ten to twelve at the front margin ten lines from the beak; cardinal area in receiving valve very large, 
flat, nearly half as high as wide, inclining backwards at 130°; foramen narrow, triangular, entirely open; area 
of entering valve flat, triangular, about seven times wider than high: internal cast of receiving valve with 
strong dental lamellze, diverging at 50°, forming the posterior lateral boundaries of a narrow, ovate, strongly- 
defined pair of muscular impressions, reaching half the length of the shell, not divided by any mesial septum, 
but having one or two strong sulci on each side; margin finely suleated by the external strise, faint extensions 
of which reach a variable distance towards the beak: cast of entering valve very tumid, nearly smooth, deeply 
slit towards the beak by the very deep narrow sulcus left by a thick mesial septum, reaching half the length 
