232 
I may have made some mistake, which those who possess more illus- 
trative specimens may be able to correct. 
It becomes here necessary to notice the specimen Plate XYI. 
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 trans- 
verse 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 
inquiry ; in the meanwhile, the conjecture on which we may rest, per- 
haps, 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. 11, the beaks are too much inverted 
to have admitted the passage of any tube or cartilage ; and I am almost 
