BIVALVIA. 129 



destitute of ornament, and have only irregular concentric plications. The anterior border 

 beneath the short anterior wing is but little excavated, its aperture being very narrow ; the 

 lower side of the shell has not much obliquity, and its border is regularly rounded. 

 Two ribs extend the length of hinge border immediately beneath it, as is usual in the 

 Pteropernse. 



In size it equals the larger specimens of our P. costatula, but it is less oblique than 

 that species : the left valve is much less convex, and the anterior sinuation is much less 

 considerable; the umbones are smaller, and are much less elevated above the hinge 

 border ; the anterior auricle is nearly upon the same plane as the posterior, but in P. 

 costatula it is directed obliquely downwards and forwards. 



Geological position and locality. — The Grey Limestone of the Scarborough Great 

 Oolite. 



Avicula Munsteri, Goldf. Tab. XIV, fig. 6. 



Testa (valva major) ovatd, obliqud, subconvexd, aid anticd acuta, posticd falciformi; 

 costis radiantibus (3-4) acutis lineisque interstitialibus incequalibus. 



Shell very oblique and convex, inequivalve ; anterior auricle acute ; posterior auricle 

 more lengthened and falciform; the larger valve with regular equal radiating slightly 

 knotted costsc (about 16 in number); in the middle of each interstitial space is an 

 elevated line, with one or more delicate or more faintly marked, upon each side of it ; the 

 auricles are ornamented in a similar manner. 



An elegant shell, with convex prominent umbones, narrow but well marked costae, 

 which slightly project at the inferior border. 



Geological position and locality. — The Great Oolite of Scarborough, in dark grey 

 argillaceous sandstone. 



Avicula Braamburiensis, Sow. Tab. XV, fig. 7, var. fig. 6. 



Avicula Braamburiensis, Sow. Geol. Trans, vol. ii, p. 323. 

 — Braamburiensis, Phil. Geol. York., 1, t. 6, f. 6. 



Testa ovato obliqua, aid anticd rotundatd, posticd obtusianguld, valvd majord convexd, 

 lineis radiantibus confertis minoribus alternis, interstiis angustis tegulatis. Valvd minora 

 convexo-pland laevigata, lineis radiant i s paiccis distantibus subobsoletis. 



Shell ovately oblique; anterior auricle small, rounded, posterior auricle forming an 

 obtuse angle ; the larger valve convex, with numerous radiating lines, alternating with 

 others which are smaller and indistinctly marked; the interstitial spaces narrow and 

 indistinctly tegulated. The smaller valve slightly convex, smooth, with a few (7) 

 radiating lines faintly marked. 



17 



