Bracutopopa. | LOWER PALAOZOIC MOLLUSGA. PNY, 
95 
pressions of the external striee, Average length six and half lines, proportional width , length of entering 
valve =, depth of both valves =. Shell under the lens granulo-punctate. 
The obtuse subequal strive, not grouped in large bundles. and the few, fine, mesial strice, easily separate 
this, as M. de Verneuil has shewn, from the O. parva of the lower strata, with which it has often been con- 
founded. The length increases considerably in proportion in old specimens, and the depth, owing to the 
curvature of the receiving valve, is sometimes (though very rarely) as low as ;% in Gothland specimens. The 
specimens of var. a. (antiquior) from Sowerby’s localities for his figure, t. 20. f. 8. are rather wider and flatter 
than those in the Wenlock limestone and Gothland strata: the lateral strize do not arch so much, the few finer 
ones are not distinct in the middle, and there is rather a greater difference in size between the anterior 
and posterior pair of muscular impressions ; still the striz and slight variations of form may be so nearly 
paralleled in a handful of Gothland specimens, that although there may be a distinction, | do not feel able 
to express it. 
Position and Locality—~-Coarse schists of Moel Seisiog, Llanrwst, Denbighshire; very common in the 
Bala limestone of Corwen, Merionethshire ; Bala schists of hills N. of Conway Falls; Bala schists of Mathy- 
rafal, Meifod, Montgomeryshire ; calcareous Bala schists, Maes y Fallen, Bala, Merionethshire; common in the 
Wenlock limestone of Dudley, Staffordshire; abundant in Bala schists of Bryn Eithin, Penmachno; sand- 
stone of Mulock Quarry, Dalquorhan; Bala schists S. of Capel Garmon, Denbighshire; Bala schists of Cefn 
y Coed, Llangedwyn, Glyn Ceiriog, Montgomeryshire; Bala schists of Yspatty Evan, N. Wales; Bala schists 
at hills S. of Cwm Lanerch, Bettws, Caernarvonshire; Upper Silurian schists of Mynydd y Gaer, S. side, 
Llanefydd, near Ruthin, Denbighshire; Bala schists of Cader Dinmael, near Corwen, Denbighshire; Bala 
schists of Blain y Cwm (at junction), W. of Nantyre, Glyn Ceiriog. S. of Llangollen, Denbighshire; Bala 
schists of. Pont y Glyn Diffwys, W. of Corwen, Merionethshire ; Bala schists of Beaver’s Grove, Bettws-y- 
coed, N. Wales; Bala limestone of Tyn y fridd, near Mathyrafal ; Bala schists of Gaerfawr, Montgomeryshire. 
Var. a. (antiquior) sandy Bala schists of Pwllheli, Caernarvonshire; Bala schists of Llwyn y ci, N. W. of 
Bala; fine Caradoe sandstone of Acton Scott, Church Stretton, Shropshire; Pala schists W-. of Llanfechan, 
Montgomeryshire ; Bala schists of Gelli Grin, Bala, Merionethshire; Caradoc schists of Plas Madoc, N. of 
Llanrwst ; near Girvan, Ayrshire; abundant in and almost composing a mass in Caradoc limestone and sand- 
stone of Horderly; in the hard ecaleareous Bala schists of Capel Curig; Upper Bala schists of Bwleh y 
Groes, S. of Bala, Merionethshire; Bala schists of Bryn Melyn, near Bala, Merionethshire ; very abundant 
in the Bala schists of Llansantfraid, Glyn Ceiriog, Denbighshire; Bala schists of Rhosfawr, N. of Glog, 
Llanfyllin, N. Wales; Bala schists of Milltir Cerrig, Llangynnog, Montgomeryshire; Bala schists of Cefn 
Coedog, Corwen, Merionethshire; schists of Bwlch y Ciban; Bala schists of Rhiwargor, near Llanwddyn, 
Montgomeryshire; Bala schists of Cwm of the Cymmerig, EK. of Bala, Merionethshire, schists of Pen Cerrig 
Serth, two miles N. of Builth, Radnorshire; common in the Bala limestone of Bala; slates of Cefn Coedog, 
Jorwen, N. Wales; Bala limestone and sandstone of Alt yr Anker, Meifod, Montgomeryshire; Bala schists of 
Coed y Bedw, Bala, Merionethshire; sandstone of Mulock Quarry, Dalquorhan, Ayrshire; Pala schists of 
Bwlch y Groes, S. of Bala, Merionethshire. 
ORTHIS EXPANSA (So07.) 
Ref. and Syn.= O. expansa Sow. Sil. Syst. t. 20. f. 14. + Orthis pecten Sil. Syst. t. 21. f. 9. (not of Dal.) 
+ Orthis alternata* Sow. Sil. Syst. t. 19. f. 6. 
Sp. Ch.—Semielliptical ; hinge-line as wide as the shell ; cardinal angles nearly rectangular ; receiving valve 
flattened, slightly convex towards the beak, gradually flattening towards the margins, which are in one plane; 
* All the specimens of this species which I haye examined from Sowerby’s localities were exterior casts of the same 
shell of which his O. eapansa was the internal cast. The outline varies from his figure of O. alternata to that of the 
specifically identical shell which he has figured as the O. pecten of Dalman. A few specimens with the proportional length 
only 7, and with a few strong concentric waves of growth at the margin, agreeing with my O. pseudopecten (Sil. Foss. Irel. 
p- 33. t. 3. f. 16.), I now believe to be only a variety of the present species. 
[Fasc. 11.] Fr 
