164 



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 opercularia, 

 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 co- 

 vered 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 XL 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 illus- 

 trate what I have here said, I have introduced Walker's figure, at 

 Plate XL- 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, not 



