THE BUILDING OF THE DERBYSHIRE LIMESTONE, 



which, by their movement, cause currents of water containing 

 food to flow down the grooves into the mouth of the 

 animal. 



On examining the minute structure of the calcareous skeleton 

 by means of a microscope it is found to consist of a fine mesh- 

 work of carbonate of lime deposited in organic tissue. In the 

 fossil forms, the interstices of the meshwork are usually filled with 

 crystalline carbonate of lime (calcite), and hence these fossils 





Fig. 5. (Two-THiRDS size of Original.) 



usually cleave in planes parallel to the faces of a rhombohedron 

 in the same manner as an ordinary crystal of calcite. Sometimes 

 the spaces are filled with silica, the calcareous substance being 

 subsequently dissolved, and frequently the minute structure is 

 obliterated. Many of the structural features described will be seen 



