THE APPEARANCE OF LIFE ON THE GLOBE 321 



very distinct from all other orders of existing 

 Crustacea, with the exception of the order of 

 Merostomata, of which one genus, the Limulus, still 

 exists in our own seas. The group of Merostomata 

 is also of very early date ; forms of great size, 

 joined under the name of Gigantostraca, lived 

 side by side with the Trilobites in the Silurian 

 seas. I shall have to return to this group also 

 when dealing with the first beginnings of life. 



But the Silurian marine fauna is not confined 

 to the Invertebrates only. Already the lower 

 Vertebrates, the Fishes at least, had appeared, 

 and figure simultaneously under three distinct 

 and sharply differentiated types. The Selachians 

 or cartilaginous Fishes carnivorous like our Sharks, 

 Ganoids with enamelled scales recalling the existing 

 Polyp tera of the Nile, and lastly the Placoderms 

 or armoured Fishes, so remarkable from their 

 elegant carapace of great bony plates protecting 

 the head and the fore part of the body. The 

 higher Vertebrates Amphibians, Reptiles, Birds, 

 and Mammals are still unknown at this epoch. 



But this picture of Silurian life would be in- 

 complete without mention of the highly important 

 fact of the first appearance of air-breathing beings. 

 In the present state of our knowledge the Arachnidae 

 of the Scorpion group alone remain to indicate 

 to us the settlement on the Silurian continents of 

 terrestrial animals with trachean respiration, seem- 

 ingly issued from marine ancestors. 



Thus the Silurian world presents itself to us as 

 a very complex, very rich, and very progressive 

 world, since we find in it beings so complicated and 

 Y 



