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It becomes here necessary to notice the specimen Plate XVI. Fig. 10 ; 

 since, in this shell, appear to be traces of such a structure, as would 

 serve to show, that the animal, in a multivalve shell of this kind, might 

 not be without the means of supporting a temporary connection, at least, 

 between these supposed cavities. 



This shell is imperforate, one valve convex and gibbous, the other 

 concave. The circumference is rounded, and the surface of each valve 

 marked with longitudinal striae, which are decussated by faint transverse 

 ones. The beak of each valve is small. A little below the beak of the 

 concave valve, a small, but apparently deep fissure, commences, which, 

 in one of the two specimens which I possess, extends in a straight line 

 towards the margin, through nearly three fourths, and in the other 

 through nearly one half of the length of the valve. This fissure has the 

 appearance of having been larger, and of having been diminished at 

 each end. In one of these specimens, the margin of the shell is entire, 

 and the valves are so exactly closed, as not to admit of any separation 

 being seen, even with a strong magnifier. In the other the margin is 

 broken, and has very much the appearance of the upper valve having 

 been produced. 



It must be left to some more successful investigator to pursue this in- 

 quiry; in- the meanwhile, the conjecture on which we may rest, perhaps, 

 with most propriety, is, that this structure has taken place for the purpose 

 of allowing the passage of some part of the animal, by which it might be 

 enabled to attach itself to other bodies. 



The shell, whose extraordinary internal structure I shall now call your 

 attention to, must .also, I believe, be placed among the imperforate shells, 

 since I have been unable to discover any aperture in the beak or hinge 

 of any of those which I have sacrificed to my inquiries ; and since, indeed, 

 in the specimen Plate XVI. Fig. 1 1 , the beaks are too much inverted 

 to have admitted the passage of any tube or cartilage ; and I am almost 



