The -&>'<t of Invertebrates 



one function of life. Besides being the highest 

 types of early life, trilobites are the most im- 



Puradoxides harlani, a typical 

 trilobite of the Cambrian. 

 (After Meek.) 



Olenellus thompsoni, a typical 

 trilobite of the Cambrian. 

 About half natural size. 

 (After Walcott.) 



portant of the early fossils in another way, and 

 that is, for the identification of strata and the 

 marking of some periods of geological time. 

 Certain genera endured for quite definite pe- 

 riods, so that they are characteristic of the 

 strata that were then being deposited, and 

 their names have been applied to the assem- 

 blage of animals, termed a fauna, found in 

 these beds. Thus the Upper Cambrian of 

 Europe contains the Olemis fauna, this genus 



67 



