172 RELICS OF PRIMEVAL LIFE 



Laurentian fossil. The first analogy which sug- 

 gested itself to Sir W. Logan, and a very natural 

 one, was that to the so-called layer-corals (Figs. 43 

 to 45) that abound in the Silurian, Ordovician, and 

 Cambrian rocks, and which though undoubtedly 



Fig. 43. —Stromatocerium 7iigosumy Hall, Ordovician. 



fossil animals, have proved very difficult to inter- 

 pret or to assign to any known group. At first 

 vaguely associated with the true corals, they were 

 subsequently regarded as probably of more simple 

 character, and as gigantic Protozoa ; and later 



