THE BEGINNING OF LIFE ON THE EARTH. 35 



tian. The species Eozoon Bavaricwn has, however, been 

 found in rocks apparently of Huronian age ; but this is the last 

 known appearance of Eozoon, properly so-called. In the 

 Cambrian or Siluro-Cambrian, however, we meet with many 

 gigantic Protozoa, more especially those ,vnovvn as Stromatopora ^ 

 ArchcBocyathuSy and Receptaadites. 

 The typical Stromatoporae, or Layer-corals, consist, like 



Fig. 2a. — Stromatopora concetitrica. — ^After Hall. 



a. Section of the same, magnified, b. Small portion highly magnified, showing lamina: 



and pillars. 



Eozoon, of concentric layers, connected by numerous pillars, 

 which are often, though not always, more definite and regular 

 than in the Laurentian fossil. The laminae are perforated, but 

 more coarsely than in Eozoon, and they are often thickened with 

 supplemental deposit which, in some of the forms, presents canals 

 radiating from vertical tubes or bundles of tubes penetrating 

 the mass (Figs. 22, 23). The mode of growth of Stromatopora 

 must have precisely resembled that of Eozoon, and the forms 



D 2 



