TRILOBITES 



381 



Asteroids (star-fishes) and Ophiuroids (brittle-stars), which had 

 likewise come in before the end of the Cambrian, increase 

 in the Ordovician. A 

 new order of Echino- 

 derms, the Echinoidea, 

 or Sea-urchins, first ap- 

 pear in the Ordovician, 

 being represented by very 

 primitive forms. 



Arthropoda. — The 

 Trilobites increase very 

 greatly in the number of 

 genera and species, and 

 most of the Cambrian 

 genera are replaced by 

 new ones. This is the 

 period in which the group 

 of Trilobites attains its 

 highest development, 

 gradually declining after- 

 ward and becoming extinct 

 with the close of the Palae- 

 ozoic. The most charac- 

 teristic and widely spread 

 genera of Ordovician Tri- 

 lobites are : Asaphus (PI. 

 II, Fig. 13), Illcenus, Tri- 

 arthrus (PI. II, Fig. 15), 

 Calymene (II, 14), Trinncleus (II, 16) 



FIG. 138. — Ordovician Trilobite. Trlarthrus 

 Becki, enlarged restoration showing appendages. 

 (Beecher.) Utica Stage, New York. 



Dalmanites, etc. These 

 genera differ in aspect from those of the Cambrian in their much 

 larger tail-shields, in their ability to roll themselves up (see II, 

 14), and in their larger and better developed, faceted eyes. 



Other Crustacea mark great advances in the Ordovician. Thus, 

 in the upper part of the system we find the first of the Cirripedia, 

 or Barnacles, a degenerate, sedentary type, and the first of the 

 Eurypterida, a group which is destined to a remarkable develop- 



