05 
ARCA. 
Gen. Char. A transverse inequilateral bivalve^ 
the beaks distant^ hinge with many teeth^ dis- 
posed in a straight line; the teeth lameliated^ 
close,, and alternately inserted between each 
other: a subrhomb oidal smooth area between 
the beaks,, on which the cartilage is disposed. 
ARCA subacuta. 
TAB, XLIV . — Upper figure . 
Spec. Char. Gibbose^ margin an obtuse scalene 
triangle deeply plaited; breadth grater than 
the length., surface longitudinally striated. 
'The hinge extends nearly the whole width of the shell, 
forming the longest side of the triangular margin ; the teeth 
are sharp and numerous ; the marginal plaits are rounded 
and very deep, particularly at the anterior side, they are 
elongated over the surface of the shell, at letist I presume 
so from the impression of the inside, which is all I have to 
examine. 
This happens to show the nature of the hinge teeth ex- 
ternally, else it might have been considered as a Cucullaea, 
a division or genus of Lamarcke’s taken out of the Area of 
Linn, and on which the teeth are on a line parallel with the 
hinge, and which I shall have an opportunity to show here- 
after. It is little else than the chalky remains of the shell 
covering a hardish Limestone Marie. From Sussex, by 
favour of Mr. Mantell. 
