114 



HISTORICAL GEOLOGY 



nearly spherical forms with deep grooves was particularly promi- 

 nent. 



Coelenterates. — Graptolites, though greatly diminished in 

 importance, were still fairly common. The more complex colonies, 



such as branching forms 

 and those with double 

 rows of cells on their 

 axes, were nearly ex- 

 tinct, the simple forms 

 mostly only remaining. 

 Anthozoans (Corals) 

 increased in prominence 

 to a very notable de- 

 gree, and the simple 

 Cup Corals (Fig. 62a) 

 of the Ordovician were 

 superseded in impor- 

 tance by the coloniz- 

 ing or compound forms. 

 Chain Corah (Fig. 62c), 

 which were rare in 

 the Ordovician, reached 

 their climax of de- 

 velopment, but became 

 nearly extinct by the 

 close of the period. 

 Honeycomb Corals (Fig. 

 62b) were also common. 

 Echinoderms. — 

 Though the Cystoids 

 reached their climax in 

 the Ordovician, they 

 were still abundant in 

 the Silurian, the Ni- 

 agara limestone near 

 Chicago being particularly rich in them. Many were unusually 

 large, and some showed greater degree of symmetry in arrange- 

 ment of plates than before (Fig. 63a). 



Blastoids still remained rare, only two genera being known 

 (Fig. 63b). 



a c 



Fig. 63 

 Silurian Echinoderms: a, Cystoid, Caryocrinus 

 ornatus; b, Blastoid, Troostocrinus reinwardti; 

 c, Crinoid, Eucalyptocrinus crassus. (After 

 Say, Troost, and Hall respectively.) 



