326 



PALAEOZOIC SYSTEM OF ROCKS. 



been probably differentiated, in one direction the more perfect Trilo- 

 bites, and in the other the Limuloids. 



Eurypterids. — In the Upper Silurian was introduced and continued 

 to exist along with Trilobites, during the rest of the Palaeozoic, another 

 family of huge Entomostracans probably in advance of Trilobites in 

 organization, viz., Eurypterids. The family includes the two genera 

 Eurypterus (broad wing) and Pterygotus (winged ear). Some of the 

 latter are the largest crustaceans known. The huge Japan crab (Ina- 

 clius Koempferi), with carapace sixteen inches in diameter, and legs 

 four feet long, and the Moluccas king-crab {Limulus Moluccanus), 

 three feet long and eighteen inches across the carapace, are the largest 

 crustaceans now living. But the Eurypterids were some of them far 

 greater. The Pterygotus Anglicus (Fig. 384) was six feet long and 

 one foot wide, and the Pterygotus Gigas seven feet long and propor- 

 tionately wide. The above figures represent some species of these two 

 genera from the American and English rocks. 



Anticipations of the Next Age. — There are some plants and animals 

 still higher than those mentioned above, but they are so rare that it is 



best to treat them as anticipations 

 of the next age. The most impor- 

 tant of these may be briefly noted : 



1. A few very small land-plants 

 (Ferns and Club-Mosses) have 

 been found in the Middle Silurian 

 of this country and of Europe. 



2. A few small air-breathers (in- 

 sects, Blattidae and Scorpions) have 

 been found in the UjDper Silurian 

 — also of both countries. We give 

 a figure of one of these very im- 

 portant discoveries (Fig. 3S7). 

 That they were really air-breathers 

 is shown by the spiracles or breath- 

 ing-pores, a. 3. A few small, curi- 

 ously-formed fishes, of very low 

 organization, somewhat similar to 

 some (Pteraspis) in the Lower De- 

 vonian, have recently been found 

 as low as the Clinton group (lower 

 part of the Upper Silurian). Such 

 anticipations are in accordance 

 with the law already mentioned (p. 



280), that the characteristics of an age often commence in the preced- 

 ing age. It is better, however, to treat of these classes in connection 



Pig. 387.— Palaeopternus— a Fossil Scorpion from 

 Upper Silurian of Scotland (after Peach). 



