THE ORDOVICIAN PERIOD 



431 



Fig. 401. — Ordovician crinoids : A, Ectenocrinus grandis ; B, Hybocrinus tumidus ; 

 C, Glyplocrinus decadactylus ; D, Heterocrinus (Iocrinus) subcrassus. 



species and long and many-branched in others. Within the arms is 

 the mouth, to which food particles are carried by the currents set in 

 motion by the arms. 



Blastoids, Starfish (Fig. 402), Brittle Stars, and Sea Urchins lived 

 in this period, but as they were rare they will be discussed in later 

 chapters. The origin of the starfish probably 

 goes back to the Proterozoic, as may be inferred 

 from the complex metamorphism of the starfish 

 larva. The absence of fossil starfish in the 

 Cambrian sediments may mean that a pre- 

 servable starfish skeleton was not evolved 

 until the Ordovician. 



Molluscoidea 



Brachiopods. — The preponderance of hinged 

 . r r » p IG ^ Q2 — Ordovician 



(articulate) (tig. 403, except C and L) species starfish : ' p a laaster simplex. 

 of brachiopods over hingeless (inarticulate) 



(Fig. 403 C and L) is very marked in the Ordovician. A con- 

 spicuous feature of many of the species was the greater thickness of 

 the shells and the ribbing (Fig. 403, F, G, H) of the exterior by 

 means of which the shell was strengthened. Brachiopods were ver} r 

 abundant and are important in determining the subdivisions of the 

 series. 



CLELAND GEOL. — 28 



