FOSSIL MOLL USCA J8IVAL VES AND UNIVAL VES. 24 i 



and arragonitic. To the modern student of mollusca 

 this may not seem of the same importance it is to the 

 geologist. To the latter, this difference in the compo- 

 sition of the limy shells of bivalves is frequently of 

 great value : for the valves composed of the limy 

 mineral arragonite are liable to be decomposed by 

 the action of percolating water through the rocks in 

 which they are imbedded much sooner than those 

 constructed of calcite. Hence the geological student 

 finds the remains of fossil mollusca possessed of shells 

 composed of arragonite chiefly, if not entirely, as 

 casts. Calcite is a more stable or endurable form of 

 lime than arragonite hence the reason why mollusca 

 whose shells are formed of arragonite are found as 

 casts, whilst those whose shells are of calcite appear 

 to be unchanged. 



Shells are useful to the geologist, also, in quietly 

 but emphatically convincing him of the former con- 

 ditions of marine and fresh-water deposition of strata. 

 He finds them bored by marine sponges. Or they 

 may be covered, inside and out, with the tubes of 

 marine worms or Polyzoa, or spat of ancient oysters 

 in which case he knows these fossil bivalves died, 

 and their parts separated, before the contemporary 

 creatures which made use of them for a mechanical 

 foothold could spread there. Such conditions quite 

 correspond to those he sees going on nowadays at the 

 seaside. 



Bivalves and univalves are the most abundant of 



R 



