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with linear chambers, adapted to the curves of the shells ; the last 
being longest. The axis marginal. 
The fossil which is represented from Lamarck, Renulinites opercu- 
laria, Plate X. Fig. 21, is the only species of this genus with which 
he appears to have been acquainted, and is about six lines in length. 
Whilst viewing this shell, he remarks, we might conceive that it 
was a very thin, fragile, flat, semilunar operculum, with the surface 
covered with parallel bowed grooves ; but, on examination, it will 
be found that it is composed of two plates, placed close against each 
other, with their inner surfaces hollowed out in contiguous bowed 
grooves, which, as the plates are applied to each other, form distinct 
chambers. This, it is justly observed, is not the kind of structure, 
observable in any operculum whatever. 
Notwithstanding the different appearances exhibited by Lamarck’s 
figures of M. opposita, Plate XI. Fig. 20, and the figure of Renulinites, 
I am very much disposed to believe them to be of the same genus, if 
not the same species. This notion I have been led to, by observing 
the figure of Walker’s bellied semilunar worm-shell, which indeed 
seems to unite the two figures of Lamarck, by possessing the general 
form of M. opposita, and the contiguous bowed grooves of Renulinites. 
To illustrate what I have here said, I have introduced Walker’s 
figure, at Plate XI. Fig. 22. 
XCI. Gyrogonites. A sphaeroidal hollow shell, composed of linear 
curved pieces, slightly grooved at the sides, where they are joined ; by 
the joining of which grooves, linear chambers, following the direction 
of the pieces, appear to be formed. At these joinings, on the external 
surface, are carinated ribs, disposed transversely about the middle, 
and spirally at each pole of the shell. At one of the poles there is 
sometimes to be seen a circular opening, which sometimes appears 
to be closed by a particular valve or operculum. 
This shell is represented magnified, Plate XI. Fig. 23, its natural 
size being that of a small pin’s head. It is found in silicious stones, 
