FOSSIL INVERTEBRATED ANIMALS. 489 



Other localities neighbouring to the same strata. The gryphites 

 are excessively numerous in some of these places, and some- 

 times so crowded and so little adherent, that they have been 

 used to pave the roads in the neighbourhood of Chavagnac, 

 near Terrasson, in the department of Dordogne, These shells, 

 which approximate to the oysters, like them have existed in 

 large famihes, for they form entire banks of three, four, and 

 sometimes six feet in thickness. They are often accompanied 

 by other shells, the living analogues of which are not known, 

 such as belemnites, ammonites, &c. Pectines, terebratulse, and 

 some other shells, are also equally associated with them in the 

 different places just mentioned, and probably in all those which 

 are analogous to them, in England, in Germany, in Belgium, 

 &c. A sort of argillaceous limestone, very much employed at 

 Lyons, contains an immense quantity of gryphites, and it is 

 well known how near to this great city is the pit-coal formation. 

 From all tliis we are led to believe, that the gryphites, which 

 are so rare in the living strata, formerly existed in great 

 abundance, towards the period when the pit-coal strata were 

 formed. A great number of varieties are known. 



This genus presents considerable difficulty in the characte- 

 rizing of species, because it blends invariably with other genera 

 which approximate to it. 



Of the univalve shells, the genus Cerithium is that which 

 presents by far the greatest number of species in the fossil 

 state. M. de France, in 1817, had already more than one 

 hundred in his splendid collection. Almost the whole of these 

 species are found in the most recent strata, and the naturalist 

 just mentioned has never seen any properly characterized 

 which were found in the most ancient strata. M. de Gerville, 

 however, in the " Journal de Physique," mentions, that in the 

 formation with ammonites and belemnites of the environs of 

 Bayeux, there were found four species of cerithia. 



Though contrary to our plan, we shall notice here one 

 species which is too remarkable to be passed over : this is 



