CAMBRO-SILURIA.N FOSSILS. 219 



are formed by the deep impressions of a sigmoidal ridge, ex- 

 tending from the cardinal teeth on each side, each impression 

 being shghtly divided into two unequal lobes, anteriorly by 

 the slight projection of an oblique narrow portion diverging 

 from the anterior lateral angles ; in fine specimens a faint 

 transverse arched furrow separates the small posterior ad- 

 ductor impressions, which are longitudinally wrinkled, from 

 the smoother and larger anterior portion ; interior of receiving 

 valve with two short cardinal teeth, diverging at 105°, from 

 which two extremely long, narrow, elliptical muscular im- 

 pressions extend more than half or nearly two-thirds the 

 length of the shell, each lobe marked with faint radiating 

 sulci, the pair being separated by a flat, acutely angular space, 

 a little wider than one of the impressions at the anterior end, 

 and extending to the point of the beak ; a narrow space of the 

 margin finely plicated, the rest of the surface nearly smooth 

 or very faintly radiated. Width 10 lines, proportional length 

 of receiving valve y%% of entering valve -^-^-q, depth y^^ . 

 The backward curving of many of the lateral striae to terminate 

 on the hinge-line is more remarkable in this species than any 

 other I know of, and suggests the specific name ; something of 

 the same sort may be seen in a less degree in the O. subquadrata 

 (Hall), and one of the shells figured in the Mem. Geol. Surv. 

 (vol. ii. pt. 1. 1. 27. f. 9) as the O. testudinaria, from both of which 

 the great general depression, the total absence of mesial fold, 

 the concavity of the entering valve, great length of the narrow 

 diverging muscular impressions in the receiving valve, &c., easily 

 distinguish the present species. The same characters and back- 

 ward curving of the lateral striae separate it from the O. protensa. 

 Occurs in extraordinary profusion in the flags of S. end of 

 Pen y gaer near Cerrig y Druidion, Holyhead Road, Denbigh- 

 shire, closely covering extensive surfaces of the beds; flags at 

 Hafod Evan, Penmachno, Caernarvonshire; schists of Cefn y 

 coed, Llangedwyn, Montgomeryshire ; schists W. side of Gam 

 Brys, S.W. of Cernioge ; abundant in the schists of Bwlch, Llan- 

 drillo, Corwen, N. Wales ; schists of Llanwddyn E. of the Berwyn 

 Mountains ; schists of Das Eithin Ridge, Hirnant, Montgomery- 

 shire ; schists of Pen Cerrig Serth, Builth, Radnorshire ; schists 

 of Miltit Cerrig, Llangynnog, Montgomeryshire; schists of 

 Bwlch y groes S. of Bala, Merionethshire; Alt yr Anker, Meifod, 

 Montgomeryshire ; schists of Tan y Bwlch y groes S. of Bala, 

 Merionethshire ; schists of Allt tre Ffynnon ; limestone of Pentre 

 cwm dda, S. of Glyn Difi'wys, N. Wales ; limestone of Llanfyllin, 

 Montgomeryshire ; common in the quartzite of Cam Goran, Corn- 

 wall. 



{CoL University of Cambridge.) 



