OF CENTRAL CANADA PART IV. 



261 



The Eurypteridae are sometimes known as Gigantostraca from the 

 large size presented by some examples. The head is covered by a 

 single plate, and carries on the under side several pairs of mastica- 

 ting, and a single pair of swimming feet. The long thorax and abdo- 

 men (including the telson) consist of thirteen moveable segments. 

 The principal genera comprise Eurypterus 

 and Pterygotus, in both of which (although 

 only seen in well preserved examples) the 

 surface of the shelly covering shews scale- 

 like markings. In Eurypterus, the an- 

 terior feet are slender and antennae-like ; 

 whilst in Pterygotus the front pair are 

 very long, and are terminated by claws 

 or nippers. In Eurypterus, also, there 

 is a long thorn-like telson, and in Ptery- 

 gotus a comparatively short and flattened 

 terminal-segment. Examples of Euryp- 

 terus in a more or less fragmentary con- 

 dition are not uncommon in the Oriskany 

 (Devonian) Formation of South-western 

 Ontario. A restored example of E. re. 

 mipes is represented in Figure 1 80. The 

 cephalic shield and the long thorn-like telson, are the parts generally 

 found. The genus ranges from the Upper Silurian into Carboni- 

 ferous strata. Pterygotus is an Upper Silurian and Devonian type. 



6. Phyllocarida : This sub-division is of comparatively recent 

 adoption, principally from the researches of Packard and Glaus. It 

 includes the modern Nebalia (a small shrimp-like crustacean with 

 stalked eyes) and a series of extinct forms of related character but 

 much larger size. In these, there appears to be a blending of 

 characters belonging to both the lower types (often grouped together 

 under the common term of Entomostraca), and the higher forms 

 (Malacostraca) of existing crustaceans. The Cambrian Hymenocaris, 

 the Silurian and Carboniferous Ceratiocaris, and the Devonian Dithy- 

 rocaris, are the principal fossil genera. They present a large and in 

 general laterally compressed head-shield (or cephalo-thoracic shelly 

 covering) composed mostly of two pieces, and a jointed or ring-formed 

 abdomen terminating in a telson of three or several spines. EX- 



FIG. 180. 

 Eurypterus remit 



a, a, are the swimining feet. 



net 



iin 



Devonian. 



