242 OUR COMMON BRITISH FOSSILS. 



the mollusca, and their hard parts or shells are among 

 the most valuable and durable of all the " medals of 

 creation." But doubtless there were sea-slugs in the 

 ancient seas, just as there are now, but as they possess 

 no hard parts (except teeth), they have not contri- 

 buted in any notable way to fossil remains. When 

 we study the fossil mollusca, we see that the univalves 

 have been more modified than the bivalves. Mr. J. 

 Starkie Gardner has also noticed this fact. He states 

 that, as regards bivalve mollusca, there does not seem 

 to be any broad rule of progression. The contrary 

 is the case with the univalves or Gasteropods. In 

 them, says Mr. Gardner, there is a most unmistakable 

 and pronounced tendency to elongate the canal. He 

 therefore thinks that the presence in greater or less 

 numbers, or the absence, of spindle-shaped or fusi- 

 form shells possessed of lengthened canals, would be 

 an infallible test of the geological age of any group 

 of Gasteropods, from the Oolitic rocks to the Eocene. 

 Thus, the Cones and the Cowries are among the most 

 highly differentiated of Gasteropods, and they are 

 also the latest introduced. 



The mollusca have a geological value in deter- 

 mining the physical conditions of ancient seas. 

 Where bivalves abound, as in the different strata of 

 the Oolite, we have evidence of shallow water, and 

 this is usually supplemented by other facts. Not 

 unfrequently the univalves bear similar testimony, for 

 many littoral or shallow-water genera have a very 

 high geological antiquity. 



