73 
the most interesting and extraordinary masses, containing shells of 
this genus, are the calcedonic masses, which are also found in some 
part of France. 
The latter specimens are rendered particularly interesting, from 
the circumstance of the shells themselves appearing to be converted 
into a calcedonic substance. The matrix is opaque, and of a pale 
greyish colour ; whilst the shells themselves are nearly transparent, 
and of a yellowish brown colour. A close examination seems to show, 
that the matrix was composed chiefly of a gritty, calcareous, or sili- 
cious mass, which has had fluid silex diffused through it, and most 
probably at the time when the change in the substance of the shells 
was effected. But to ascertain the real nature of the change, and to 
form a rational opinion as to the mode in which the carbonate of lime 
has been removed, or so changed as not to prevent the transparency 
of the fossil shells, I confess myself entirely incompetent. 
XXX VI. Trochus. A conical spiral univalve, with a transversely 
depressed and rather quadrangular opening ; with an oblique axis. 
Lamarck has separated from the genus Trochus, of Linnaeus, those 
shells which possess such peculiar characters as warrant their being 
arranged under two new genera, which we shall presently notice ; 
Solarium and Monodonta. 
At Grignon is found the remarkable carrier trochus, T. agglutinans, 
Lam. T. conchy liophorus, Lin. distinguished by its curious property ot 
attaching and agglutinating to its surface small shells, pebbles, frag- 
ments of coral, &c. M. Lamarck gives the figure of one of these fossil 
shells, loaded with small bivalves ; but none of the specimens which I 
possess are loaded: they, however, show sufficient marks of the attach- 
ment of numerous foreign bodies, and even retain the impressions of 
the very minute markings of small shells which had been affixed to 
them. A specimen of this species of shell which I possess, from 
Verona, is remarkable for its size ; the diameter of the base, and of 
the side of the cone, each measuring above three inches. 
VOL. III. 
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