OUR COMMON BRITISH FOSSILS. 



flints the student may be sure to find abundance 

 of fossil sponges. The hollow ones should have 

 their interior contents carefully- 

 boxed for subsequent examina- 

 tion. I have frequently carried 

 as much "flint-meal" away in 

 a small pill-box as afforded a 

 week's work with the microscope; 

 Many flints are called "rot- 

 ten " by the quarrymen because 

 they are permeated by irregular 

 canals, branching off from each 

 other like the twigs of a tree. 

 Sometimes these canals are filled 

 with "flint-meal," which should 

 be carefully collected. The sur- 

 faces of the walls of these branch- 

 ing canals or passages are always 

 rough. It does not require much 

 acumen to see that these are 

 due to fossil-branching sponges, 

 ,^^. resembling in shape the Chalinas 



"^^^^^aX %^ and HalicJiondrias of our British 

 * I ^^ seas. All geologists are now 



agreed that the flints of the 

 chalk formation somehow as- 

 sumed their present shapes, appearance, and distri- 

 bution through the agency of crops of sponges which 

 flourished on the ancient Cretaceous sea-bed, Con- 



Fig. II. — Common Fossil 

 Sponge from Greensand 

 {Siphonia pyriforinis). 



