MATERIALS 15 



which the organism belongs, it is logical to surmise 

 that the earlier members of shell-bearing orders will 

 have been relatively, or even absolutely, soft-bodied. 

 As far as the evidence goes, this assumption seems 

 correct. The early Molluscs and Brachiopods had very 

 delicate shells containing much chitin, while many later 

 members of those groups possess very massive cal- 

 careous coverings. It is at least possible'that the perish- 

 able nature of early representatives of Invertebrate phyla 

 may account for the lack of evidence of the evolution 

 of great groups from one another. This question will 

 be discussed more fully in Part II. Chapter I. 



The habitat and normal mode of life of an animal 

 strongly affect its chances of fossilization, apart from the 

 influence they may have on the development of its 

 skeletal structures. Early and effective burial of the 

 remains is essential for preservation of the most massive 

 mineral fabric; for rain-water, temperature-change and 

 bacterial action disintegrate exposed shells even more 

 rapidly than they destroy the rocks. Since it is more 

 usual for detrital rock-matter to be removed from a 

 land surface than for it to accumulate there (save for 

 temporary stoppages in the transport system), the 

 opportunities for rapid or permanent burial of terrestrial 

 or fluviatile animals are rare. With few exceptions. 

 Invertebrates that live in the air, on land, or in fresh- 

 water, build far less solid hard-parts than those inhabit- 

 ing the sea. This is partly due to the frequent lack of 

 dissolved mineral-matter available for the former kinds, 

 and partly because of the need for lightness and agility. 

 Unless the remains of such animals become quickly 

 drifted to a suitable burying-place after death, their 

 permanent preservation must be very exceptional. The 

 extreme scarcity of fossils in such terrestrial or inland- 

 water deposits as the Old Red Sandstone, the Trias, 



