Different species of this genus are found, in delicately beautiful cal- 

 careous masses, in the neighbourhood of Courtagnon. But among the 

 most interesting and extraordinary masses, containing shells of this 

 genus, are the calcedonic masses, which are ajso 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 cal- 

 cedonic 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 silicious 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. 



XXXVI. 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 ; So- 

 larium and Monodonta. 



At Grignon is found the remarkable carrier trochus, T. agglutinans, 

 Lam. T. conchyliophorus, Lin. distinguished by its curious property of 

 attaching and agglutinating to its surface small shells, pebbles, fragments 

 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 attachment 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 re- 



VOL. in. L 



