104 
conspicuous plaited or sharp zigzag undulations, but this 
might be traced by an attentive selection of specimens by 
degrees to the common oysters. This being the case, I may 
perhaps err when I make this shiell distinct from O. diluviana 
Linn. Our shell does not very clearly show the erect 
acutangular teeth, but is rather more irregular, and the 
many specimens [ have examined, have as many various — 
figures, being rather oblique chiefly to the right, but often 
to the left; wider than long; roundish, semi-lunate or 
ovate; more or less imbricated, sometimes very deeply ; 
the inside varying in depth, and the cicatrix or muscular 
impression very variable. They are gregarious, parasitical, 
or independent. 
The upper specimen is bleached and weathered, as it 
were, by exposure, differing but little probably in its sub- 
stance from a recent dead oyster shell, that has been exposed 
some time on the sea shore. The lower specimen is less 
altered; the ins*Je of one valve is shown to. expose the 
muscular impression, the other shows a concave furrow of 
adhesion near the hinge, which is scarcely perceptible in 
our specimens. 
I name this after an assiduous investigator, the Rev. 
T. O. Marsh of Felmersham, in whose neighbourhood it is 
found abundantly in all its varieties, and to whom I am 
obliged for specimens. 
