350 ARACHNIDA. EURYPTERIDA 



ments of the mesosoma broader than those of the metasoma. The 

 tail-plate is oval, ending in a pointed process or spine. Metastoma 

 heart-shaped. The pre-oral appendages (chelicerae) are small ; the 

 second pair of appendages are slender, and composed of six joints ; 

 the third, fourth, and fifth pairs have seven joints, and are similar 

 in size and form ; the sixth pair are longer and have a large retort- 

 shaped basal joint. Upper Ludlow and Passage Beds. Ex. S. 

 acuminata, Uppermost Silurian. 



Distribution of the Eurypterida 



This Order ranges from the Cambrian to the Permian, 

 but is most abundant in the Upper Silurian and the Old 

 Red Sandstone. The only form known from the Cambrian 

 is Strabops, from Missouri ; from the Ordovician, Echino- 

 gnathus. The chief genera in the Silurian and Old Red 

 Sandstone are Eurypterus, Stylonurus, Pterygotus, Hugh- 

 milleria, and Slimonia; and in the Carboniferous and 

 Permian, Eurypterus. 



SUB-CLASS II. EU ARACHNIDA 



The Euarachnids breathe air by means of either pul- 

 monary sacs or tracheae, and the mesosoma is without plate- 

 like appendages. The principal Orders are: — (1) Scor- 

 pionida, (2) Pedipalpi, (3) Araneida, (4) Pseudoscorpionida, 

 (5) Phalangidea, (6) Acarina. 



ORDER I. SCORPIONIDA 



The Scorpions (fig. 150) have a long, narrow body, in 

 which three regions are clearly marked. In front, the 

 pfosoma or cephalothorax consists of six fused segments, 

 covered dorsally by a chitinous carapace which bears a 

 pair of simple eyes near its centre, and a group of simple 



