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. — Stromatoceriii77i fugosutHy 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 



