BRACHIOPODA 39 1 



sea-urchins the number of rows of plates is either more or less 

 than twenty ; in some of the Silurian genera the plates are loosely 

 fitted, and slightly overlapping, like fish-scales. 



Arthropoda. — Among the Crustacea the Trilobites are still numer- 

 ous, though decidedly less so than they were in the Ordovician ; 

 they represent, for the most part, new species of genera which have 

 survived from the preceding period. The commonest genera are 

 Calymene, Illcenus, Dahnanites (Fig. 139/2), Lie has (Fig. 139/3) ; 

 while newly added are the genera Phaeops, Proeius, Encrinurus, 

 etc. Eurypterids continue to increase in numbers and size, though 

 not reaching their maximum in either respect until the Devonian. In 

 these extraordinary Crustacea the head is very small and is followed 

 by a long, tapering body, composed of thirteen movable segments ; 

 the last segment is either a pointed spine, as in Enrypterus, or a 

 broad tail-fin, as in Pterygotus. Five pairs of appendages are 

 attached to the head, the bases of four of which, on each side 

 of the mouth, form the jaws, as in the existing horse-shoe 

 crab. The first pair of appendages are either short and simple 

 {Eurypterus, Stylonurus), or are much elongated, and armed 

 with pincers {Pterygotus) . The fifth pair are either very long, or 

 enlarged to serve as swimming paddles. The first body-segment 

 carries a pair of apron-like appendages, with a narrow median 

 extension, but the other segments have no appendages. The 

 horse-shoe crabs find their most ancient representative in the 

 genus Hemiaspis of the European Silurian. Other Crustacea are 

 much as in the preceding period. 



Scorpions have been found in the Silurian of Europe and America, 

 and some remains oiLnsecis in the former continent. These animals 

 prove the existence of a contemporaneous land vegetation, and con- 

 firm the doubtful evidence of the Ordovician and Silurian plants. 



Bryozoa are quite abundant, and contribute in an important way 

 to the growth of the coral reefs. 



Brachiopoda continue to be present in multitudes, but with a 

 distinct change in dominant genera from those which were com- 

 monest in the Ordovician. Especially characteristic is the increase 

 in the families of the Spiriferidez, Pentameridce, and Produetidee, all 



