32 
base, and half way over the posterior side, when they be- 
come obsolete, but appear again close to the beak; the 
inferior edge is serrated so as to resemble an Arca.* The 
superior margin is semicircular. The coat looks like an epi- 
dermis; the inner coat is often richly pearly and iridescent. 
Highgate has produced this species in great abundance, 
in very large clusters much crowded and jumbled together, 
from half an inch to two inches in breadth. They are very 
apt to scale into laminz coat after coat, till the surface is 
altogether pearly. I had a fine specimen found about 300 
feet deep in the clay bed in Richmond Park, but the clay 
which I dried very carefully, yet shrunk from the shell, 
so that only the impression remains. 
Bognor, in Sussex, affords this shell, I believe, but 
rarely, the specimen figured at the bottom of the plate is 
perhaps distorted by some accident; if not, it might be dis- 
tinguished by its peculiar contour, as it is somewhat thicker 
than wide, with the posterior side depressed, which gives it 
a bow-like curvature. I had this specimen by favour of a 
great friend to the science, W. Boys, Esq. F,L.S. 
* In Mytilus Bidens, which this shell much resembles, the serratures on 
the hinge are very prominent, and pass ail round the edge of the shell, and 
are distinctly marked at the beak. Ours appears also to have hinge teeth 
at the beak. 
