112 Evolution and Distribution of Fishes 



odus. or Lanarkia (Fig. 8a, 8b) consisted of scattered 

 tubercles embedded in a firm skin. More extensive deposi- 

 tion of calcium carbonate gave rise to such armored types 

 as Cyathaspis, Auchenaspis and Cephalaspis. Isolated 

 defensive spines, referred to the genera Onchus and Cli- 

 matius suggest that primitive forerunners of the Shark 

 alliance or Selachii were already evolving in Silurian fresh- 

 waters. 



Owing to an environal combination of conditions that 

 is still often highly destructive to fish-life, but was then 

 undoubtedly greatly more active and widespread, such as 

 subaqueous earthquakes, explosions of poisonous gases, 

 widespread deposition of volcanic dust and ashes, severe 

 storms over land and lake areas, sudden torrential rise and 

 fall of the lakes, rivers, or swamps in which the fishes 



Fig. 8, a 



Fig. 8, b 



Fig. 8. — a, Restoration of Thelodus scoticus, a primitive fish 

 probably derived from a metanemertean ancestry, b, Restoration of 

 Lanarkia spinosa. (After Traquair.) 



