128 MOLLUSCA FROM THE GREAT OOLITE. 



valve is somewhat more convex ; the shining surface of the valves discloses closely ar- 

 ranged, very delicate and unequal concentric lines, which are decussated by radiating lines, 

 equally dense, but slightly waved and knotted when viewed under a magnifier; the auricles are 

 densely striated. The auricles are so small, that the length of their superior border is less 

 than a third of the height of the shell, the measurement of the lateral diameter being equal 

 to the height. The specimen forwarded to us from Yorkshire, is only 14 lines across, and 

 agrees with small examples from the Inferior Oolite of the Cotteswolds, in which latter rock 

 the species attains to thrice this measurement. 



Geological position and localities. — The Coralline Oolite of Malton, the Kelloway rock 

 of Scarborough, the Cornbrash of Gristhorpe, the Great Oolite or grey limestone at Cloughton, 

 and the bed called Trigonia Grit, in the Inferior Oolite of the Cotteswolds ; it would appear 

 to be abundant in each of these positions. 



Pern a rugosa, Gold/, var. Tab. XIV, fig. 16, et antea, Tab. Ill, fig. 1. 



Pekna quadrata, Phil. Geol. York., t. 9, f. 21, 22. 

 — — Gold/. Pet., t. 107, f. 12. 



Testa ovato-sigmoided convexo-pland, in alam brevam productd; umbonibus acutis promi- 

 nentibus ; margine cardinali obliquo, canaliculis (8 — 12) piano concavis. (Goldfuss.) 



A subquadrate thick shell, with a lengthened and large series of hinge-grooves ; the apex 

 is pointed, and projects forwards, beneath which the anterior border is concave and in- 

 crassated, the lower border is rounded, the posterior side of the shell is thin, and its border 

 nearly straight. The surface has irregular concentric plications, which, however, are not 

 very prominent. 



Aged specimens acquire a very considerable degree of elongation, the opposite measure- 

 ment upon the hinge border having but little increase, usually the figure is more quadrate 

 or less sigmoidal than is represented by Goldfuss. 



Geological position and localities. — In Yorkshire, P. rugosa, var. quadrata occurs in the 

 grey limestone of the Scarborough Great Oolite ; in the Cotteswolds, we have examples 

 both from the lower and upper division of the Inferior Oolite. 



Pteroperna plana. Tab. XIV, fig. 4. 



Testa obliqud, alatd, lined cardinali recto elongato, postico valde producto, valvis 

 subcequalibus, depressis, inornatis ; plicis concentricis irregularibus. 



Shell oblique, winged ; umbones small, acute, curved forwards, and placed near to the 

 anterior extremity of the hinge-line, above which they are scarcely elevated ; hinge border 

 lengthened, produced posteriorly into an extended and pointed wing; the valves are 

 nearly equally flattened, the left valve being a little more convex than the other ; they are 



