THE BEGINNING OF LIFE ON THE EARTH. 37 



of different types have been nearly as great rock builders 

 as they were in the Laurentian. Some of these later roclc- 

 builders, however, have belonged to the lower or imperforate 

 group ; others to the higher or Rotaline and Nummuline groups ; 

 and, as a whole, they have been individually small, making up 

 in numbers what they lacked in size. Probably the conditions 

 for enabling animals of this type rapidly, and on a large scale, 



Fig. 24. — Arc/tceocyathtis minganenst's. A Primo; Hal Protoz;on.— After Billings. 



a Pores of the inner wall. 



to collect calcareous matter, were more favourable in the 

 Laurentian than they have ever been since. 



In the Siluro-Cambrian age two other forms of gigantic 

 Foraminiferal Protozoans were introduced, widely different from 

 Eozoon, and destined apparently not to survive the period in 

 which they appeared. These were Archaeocyathus, the ancient 

 Cup-corals, and Receptaoulites, which may perhaps be called 



