201 



one side 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 

 appearance 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, disposed 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 XIV. 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 preserved. 

 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 com- 

 plicated 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, A, 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 as being of the species F. echi- 

 nata, Lam, ; but, on examination with a lens, it is discovered that the 



VOL. Ill, D D 



