336 GINGLYMACONCHA. ARCADZ. 
This species is very abundant on most of the coasts of Great 
Britain, most commonly occurring in friths and estuaries, but 
rarely on the open shores of the sea. A very thick dwarf 
variety occurs in the crevices of the rocks perforated by the 
Pholades at Hastings. 
I received from my worthy friend, John Stephenson, Esq., 
Civil Engineer, a very elegant variety of this species, taken 
from the bottom of a vessel that had for many years been an- 
chored off the Bell-rock, prior to the building of a lighthouse, 
since erected by this most ingenious engineer: these specimens 
are rather longer than those found in other parts, which I at- 
tribute to their peculiar situation: from having been suspended 
from the bottom of the vessel, they had doubtless assumed a 
more regular form than if they had been attached to rocks in 
an upright position. 
A perfect series from this ship are now in that part of my 
zoological collection which I presented to the British Museum. 
FAMILY XIII. ARCADA. 
Shelis elongate ; the valves nearly equivalve and nearly equi- 
lateral; the hinge linear and straight, with numerous teeth 
inserted alternately ; the ligament internal. 
Genus 66. ARCA. 
Shells transverse, nearly equivalve and nearly equilateral ; 
the umbones distant, separated by an area; the hinge linear 
straight, its extremities not ribbed; with numerous teeth 
placed very near together and inserted alternately ; the liga- 
ment external. 
The foot is abruptly clavated, truncated at its extremity. 
1. Arca Noa. 
Shells oblong, striated, their extremity emarginated; the 
