26 THE NAUTILUS. 
and a rather small, deep, tubular umbilicus. It is covered with 
beautiful oblique epidermal elevated ridges, which are easily lost, 
and do not agree with the lines of growth. The H. ce@ca is much 
smaller, olive-greenish, with a silky lustre and few inflated whorls 
the first of which is usually finely punctate. 
The suture is very deep and the umbilicus proportionally larger 
than in H. granum. 
ON A SINGULAR CASE OF IMITATION IN OSTREA VIRGINICA. 
BY CHAS. T. SIMPSON. 
I have before me a shell of Cerithium atratum about 18"" in 
length, which has attached to it and growing on the side of its spire 
a young Ostrea virginica about 16" in length, and 6™™ in width. 
There is nothing at all surprising in the fact that a young oyster 
should so attach itself to a Cerithium or any other shell, but it is 
surprising that the oyster should attempt to pass itself off for a part 
of the shell on which it grew. For, strangely enough, the upper valve 
of the oyster is sculptured exactly like the surface of the Cerithium. 
Each revolving ridge and nodule is repeated on the bivalve exactly 
as it is found on the spire of the shell on which it grows, just as 
perfect and distinct in every respect ; the only difference being that 
they are not quite so strongly elevated as they are on the Cerithium. 
Nor is this all. Not only is the sculpture repeated on the valve 
of the oyster, but the coloring of the Cerithium is carried over upon 
it; it being a yellowish-white throughout, covered with brown flecks 
and spots. When I first examined the shell I supposed that its 
spire had been injured, and that it had repaired it with an awkward 
patch ; but only after the closest scrutiny did I discover the truth. 
Two other very small oysters had attached themselves to other parts 
of the shell, but as their upper valves were missing at the time 
I first examined it, I could not tell whether they had been similarly 
marked or not. 
It is no uncommon thing for shells which attach themselves to 
others, to imitate those on which they grow; though I have never 
seen quite so remarkable a case as this. The shells of Anomia 
glabra and sometimes Crepidula fornicata, when growing on the 
Pecten imitate them by being ribbed, and Crepidula plana has 
often the texture of the interior or exterior of the shells on which it. 
