28 AN INTRODUCTION TO THE STUDY OF FOSSILS 



20. How do animals migrate ? 



21. Name three groups of animals whose hard parts consist 

 principally of lime carbonate. 



22. Give an example of a siHceous skeleton ; of a chitinous. 



23. What is the most usual hardening substance of plants? 



24. Mention two plants with siliceous hard parts, two with 

 calcareous hard parts. 



