PROM THE INFERIOR OOLITE. 497 



belongs to that genus. It has the same cartilage-grooves, wide 

 hinge, and thin test ; and, on the presumption that the lines between 

 the horse-shoe anterior mark, the circular posterior one, and those 

 joining the latter with the hinge in the recent shell, are lost or 

 almost imperceptible in the fossil cast, there is almost exact agree- 

 ment in the muscular impressions. 



Placttsta sagittalis, n. sp. Plate XV. fig. 17. 



Eight valve elongated, oblique, longitudinally flat, transversely 

 convex. Apex small, posterior, facing backwards, marginal, not 

 raised above the surface of the shell. Hinge-margin nearly straight, 

 the posterior part much shorter than the anterior, and inclined at 

 a slight angle to it. Shell transparent, very thin. Hinge-markings 

 visible through the shell ; two long (4 lines), narrow, oblong, carti- 

 lage ridges, meeting at an angle of about 45°, between which are 

 possibly small ones bounding a concave groove. Surface covered 

 by very crowded and fine concentric thread-like striae which become 

 waved near the margins, and more distant and regular lines of 

 growth. On the lower part are numerous very indistinct and 

 broken longitudinal swellings, and a few rows of distant cavities, 

 like reversed arrow-heads, which tend to form radiations. 



Locality. Dundry. I have only obtained a single specimen. 



Dimensions. Length 16 lines, width 16, depth 2. 



On one portion of the surface is a possibly attached fragmentary 

 surface of plain shell, covered by numerous minute ovals, which 

 may represent fry. 



Placunopsis oblonga, Laube, 'Bivalven von Balin/ 1. 1, fig. 8, closely 

 approximates to it, but is finely radiated, and differs in the size 

 and position of the umbo. 



Placuxopsis semistriata, Bean, sp. 



1839. Anomia semistriata, Bean. Mag. Nat. Hist. 1839, p. 61, 

 fig. 21. 



1863. Placunopsis .semistriatus, Lye. Suppl. Moll. t. 33. figs. 9,9a. 



I have found a specimen of this shell in the interior of Terebratula 

 perovalis at Dundry, in the same way as those described from the 

 north of England. 



Pecten arattjs, Waagen. 



1867. Pecten aratus, Waag., in Benecke's geogn.-pal. Beitr. t. 31. 

 fig. 4. 



This large, round, and flat shell is distinguished from P. lens, Sow., 

 by the multitudinous and finely radiating river-like markings of the 

 eared valve, which continually bifurcate and often disappear as they 

 approach the margins. Prom P. ductus, Sow., it is also distinguished 

 by the absence of the regular lines of growth and by its very much 

 smaller size. 



The other valve is also flat. Its central part is covered with 

 circles of verv coarse puncta, which coalesce into an irregular zigzag 



