40 INVERTEBRATE PALAEONTOLOGY 



Specimens of fossiliferous amber, probably derived 

 from older deposits, are occasionally found in the 

 East Anglian Pliocene and Pleistocene. 



The shelly structures of fossil Invertebrates are 

 commonly found in a condition hardly different from 

 those of living forms. Usually, however, complete 

 removal of the encasing and penetrating organic tissue 

 renders " desiccated " shells very friable. Since solu- 

 tion or replacement are inevitable for shells exposed 

 to the action of water, clays or nodules supply the only 

 matrices in which unchanged preservation can be long 

 maintained. Even in such surroundings the nature of 

 the shell-substance influences the permanence of the 

 fossil. Calcite may retain its original characters so 

 perfectly that its pearly lustre (and even traces of 

 coloration) may be found in deposits as old as the 

 Lias. Even in the porous sands of the Crag, Ostrea 

 persists unchanged. But Aragonite can rarely escape 

 removal or alteration, whatever may be the character 

 of its surrounding matrix. Comparison of figs. 4 and 

 5 (Plate iii.) shows the different condition of shells 

 composed of the two forms of Calcium carbonate in 

 the same deposit. Striking illustrations of the differing 

 durability of Calcite and Aragonite occur in the Chalk, 

 where large shells built of the latter (e.g. Ammonites 

 and Gastropods) are represented by casts and moulds, 

 to the walls of which adhere encrusting Polyzoa and 

 Spondyli which, being made of Calcite wholly or in 

 part, are preserved almost unchanged. 



Opaline silica has considerable permanence when 

 protected from percolating water, but it undergoes 

 " devitrification " in time, or under influence of pressure. 

 Chitin may remain very fresh if sealed up in a nodule 

 or impervious clay (PI. iv. fig. i). The Arthropoda 

 of the Coal Measures are but little carbonized when 



