THE LOWER AND UPPER SILURIAN AGES. 71 



atites which sometimes attained to gigantic size. The 

 shells of these creatures may be compared to those of 

 nautili straightened out, the chambers being placed in 

 a direct line in front of each other. A great number 

 of species have been discovered, many quite insignifi- 

 cant in size, but others as much as twelve feet in 

 length and a foot in diameter at the larger end. 

 Indeed, accounts have been given of individuals of 

 much larger growth. These large Orthoceratites were 

 the most powerful marine animals known to us in the 

 Silurian, and must have been in those days the tyrants 

 of the seas.* 



Among the crustaceans, or soft shell-fishes of the 

 Silurian, we meet with the Trilobites, continued from 

 the Primordial in great and increasing force, and 

 represented by many and beautiful species; while 

 an allied group of shell-fishes of low organization but 

 gigantic size, the Eurypterids, characteristic of the 

 Upper Silurian, were provided with powerful limbs, 

 long flexible bodies, and great eyes in the front of 

 the head, and were sometimes several feet in length. 

 Instead of being mud grovellers, like the Trilobites 

 and modern king-crabs, these Eurypterids must have 

 been swimmers, careering rapidly through the water, 

 and probably active and predaceous. There were 



* Zoologists will observe that I have, in the illustration^ 

 l^iven the Orthoceras the arms rather of a cuttle-fish than of a 

 nautilus. The form of the outer chamber of the shell, I 

 think, warrants this view of the structure of the animal, which 

 must have been formed on a very comprehensive type. 



