26 Tin: xautilus. 



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 gi-owth. The H. cceca 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 jDrojiortionally 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 lO"""- 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 oy.ster 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 Cropidula fornicata, when growing on the 

 Pecten imitate them by being ribbed, and Crepidula j^lana has 

 often the texture of the interior- or exterior of the shells on which it 



