LIFE 



387 



brian, an adjustment not greatly inferior to that which now prevails 

 amon.n the corresponding orders. Higher types within the same 

 on Ins have been developed since in many cases, but some of the 

 Ordovidan forms have since suffered degeneration. The Ordovician 

 ancestors of the barnacle, for example, free-moving, active forms, 

 were doubtless superior to their sessile descendants of ill-repute. 



Land life. But few relics of land plants, and these somewhat 

 doubtful, have been found in the system, and they reveal but little. 



The oldest relic of insect life now known is a rather obscure wing 

 found in the shales of the Upper Ordovician of Sweden. It is re- 

 ferred to the order of Hemiptera (bugs). The existence of flying 

 insects implies the presence of vegetation, and of atmospheric 

 conditions suited to active, air-breathing organisms. 



Succession of Faunas 



There was a succession of Ordovician faunas, somewhat unlike 

 OIK- another, just as there was a succession of Cambrian faunas. 

 These may be distinguished roughly as the Lower, Middle, and 

 Upper Ordovician faunas. In some places, the late Cambrian 

 and early Ordovician faunas merge into one another without 

 sharp definition. In general, the Mid-Ordovician fauna was 

 more prolific than that which preceded, if we may judge from 

 the fossils. The Mid-Ordovician fauna, too, was distinctly cos- 

 mopolitan. The Upper Ordovician fauna was similar to its prede- 

 cessor, from which it descended, but clear-water forms were less 

 dominant. 



Map work. See note at end of last chapter. The folios serviceable for the 

 Cambrian system are serviceable also for the Ordovician. See also Exercise IX 

 in Laboratory Exercises in Structural and Historical Geology. 



