2o8 OUR COMMON BRITISH FOSSILS, 



carabtiSy Dromilites (possibly a fossil hermit-crab), etc. . 

 Some of these derivative specimens are very perfect, 

 others are water-worn ; but all are imbedded in what 

 was once a phosphatic paste. Singularly enough, 

 marine phosphate deposits are frequently remarkable 

 for their fossil crustaceans ; as, for instance, the 

 Greensand " coprolites " of Cambridgeshire, where we 

 have an abundance of the carapaces of Notopocorystes, 

 In the Silurian formation we have certain thin beds 

 of phosphorite, or phosphate of lime, more or less 

 associated with Trilobites, and perhaps partly pro- 

 duced by their subsequent chemical transformation ; 

 for we have to remember that the carapaces of crusta- 

 ceans are remarkable for the quantity of phosphate of 

 lime which enters into their composition, 



