116 



A RCA duplicata 

 TAB. CCCCLXXIV— /g^. 1. 



Spec. Char. Transversely ovate-elongated, con- 

 vex, longitudinally ribbed ; ribs sulcated along 

 the middle ; margin toothed ; beaks close. 



Remarkable for having doubled or forked ribs ; the 

 surface is convex without any ridge distinguishing the 

 anterior side ; the sinus in the edge is obscure. 

 Sent from Hordwell by several friends. 



ARC A depressa. 

 TAB. CCCCLXXIV.— /g^. 2. 



Spec Char. Elongated transversely, depressed, 

 marked w^ith elevated and crenulated striae ; 

 extremities rounded ; marginal sinus obscure. 



1 HE rounded extremities, or rather sides, and distant 

 elevated strias decussated by the lines of growth, distin- 

 guish this Area ; the striae are very distant upon the an- 

 terior side, and appear like small knotted threads. 



Very much mutilated specimens of this shell were 

 picked up in 1821, among the oysters in the clay above 

 the sand, in the pits near Woolwich : it is unfortunate 

 that none more perfect have come to hand, as I am not 

 certain that its flattened form may not result from pres- 

 sure entirely. 



ARCA tumida. 

 TAB. CCCCLXXIV.— /g^. 3. 



Spec. Char. Transversely elongated, gibbose, cos- 

 tated ? anterior side pointed ; marginal sinus 

 short, deep ; beaks distant. 



Above twice as wide as long ; the depth of each valve 

 nearly equals its length ; the beaks are very near the 

 posterior extremity ; there is an obtuse ridge runs from 

 the beak to the anterior angle. As the specimens are only 

 casts of the inside, the surface cannot be described ; but 

 there are indications of rather distant furrows with ribs 

 between them upon some individuals. 



Occurs plentifully in the Magnesian Limestone of 

 Tunstal Hill, near Sunderland. 



