116 
ARCA duplicata 
TAB. CCCCLXXIV— /g. 1 
Spec. Char. Transversely ovate-elongated,, con- 
vex,, longitudinally ribbed ; ribs suicated 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. 
ARCA depressa. 
TAB. CCCCLXXIV— ^g. 2. 
Spec. Char. Elongated transversely, depressed, 
marked with elevated and crenulated striae ; 
extremities rounded ; marginal sinus obscure. 
The rounded extremities, or rather sides, and distant 
elevated striae 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. CCCCLXXLV-- jig. 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. 
