314 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Eurypterus (Fig. 136). Ordovician to Permian. 



Abdomen, including the telson, about five times as long as 

 the subquadrate cephalothorax, which is bordered by a narrow 

 furrow. Compound eyes reniform ; the simple ones between 

 these are close to the median line. First pair of head appendages 

 very small ; second to fourth pairs covered with spines ; the 

 sixth or last pair very large. 



1. Sketch Eurypterus, dorsal view, noting both simple and 

 compound eyes, cephalothorax, telson, appendages if present. 



2. Describe its habitat and probable mode of life, giving 

 reasons for your views. 



3. How can the ontogeny of an extinct animal Uke Eurypterus 

 be known ? 



4. In what respects do the larval stages of this genus differ 

 from the adult? 



5. What is the significance of the name Eurypterus ? 



Order 3, Limulava 



An imperfectly known order from the Middle Cambrian. 

 Attached to the short cephalothorax are a pair of long, simple, 

 jointed antennae similar to those of trilobites, and four pairs of 

 other appendages, some of which are likewise trilobite-like in 

 being biramous. The broad, tapering abdomen of twelve seg- 

 ments ends in a compound fin-like telson similar to that of the 

 decapod crustaceans. Compound eyes only are present. This 

 order may represent species transitional between the primitive 

 arachnidan order Eurypterida and the primitive crustacean 

 order Trilobita. The best-known genus is Sidneyia from the 

 Stephen formation of British Columbia. 



Orders 4 to 16 



(The air-breathing portion of the Arachnida) 



4. Scorpionida (Silurian to present). The telson is modified 

 to form a poison gland and sting. The scorpions have decreased 

 since their maximum development in the Upper Paleozoic. For 



