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one f. ide shows three conical lobes, the central one having a pyramidal 
form, the point terminating upwards, between the points of the two 
others, which meet like the beaks of a bivalve, yielding somewhat of 
the appea ance of a grotesque mask, or of the figure which is assumed 
by some chrysalides ; and on the point where these conical bodies 
unite is found, in some specimens, a piece of a particular form, dis- 
posed like the rotula of the knee. The opposite side of the tube 
is rounded like the back of the human head, and is marked by two 
oblique suture-like lines, which descend, and, uniting, seem to mark 
the termination of the head in the neck. 
Plate XI Y. Fig. 10, represents one of the largest-sized fossils of this 
species which I have seen, and the most contorted in its form. This, 
it is seen, has lost the rotula, which in the specimen, Fig. 8, is pre- 
served. It is somewhat of a triangular form, disposed between the 
lateral lobes, and connected with the posterior surface by a narrow, 
and apparently corrugated process. 
Whether the larger end of this shell was always closed or not, is 
very difficult to determine. It appears, however, not probable, that 
so complicated a form should have been without some use ; and the 
rotula, it is very likely, might have been employed by the animal in a 
manner somewhat similar to that in which the spatula-formed opercula 
of the teredo was used. 
In Plate XIV. Fig. 12, a, and 12, b, are represented two specimens 
of shells of this genus, from France. In the specimen Fig. 12, a, the 
sides of the tube are so removed, that the outer sides of both the 
valves are seen ; and in Fig. 12, b, the inner side of one of the valves 
is shown, set in the side of the tube. So much of the outer part of 
the tube is removed, in these specimens, as to render it difficult to 
speak of the species to which they should be referred. 
At first view, the echinated surface which they display leads to the 
supposition, that they should be considered asbeing of the species A 1 , echi- 
nata, Lam. ; but, on examination with a lens, it is discovered that the 
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