THE SILURIAN PERIOD 



115 



Crinoids very considerably increased in numbers and species 

 as well as in complexity of structure (Fig. 63c). "They attained 

 such abundance in certain localities that their fragments formed 

 the main substance of the limestone. These spots became veritable 

 'flower-beds' of 'stone lilies/ and certain localities, as Lockport, 

 N. Y., Waldron and St. Paul, Ind., Racine, Wis., Chicago, 111., 

 Gotland, Sweden, and Dudley, England, have become noted as 

 peculiarly rich crinoidal fields, where beautiful and varied forms 



Fig. 64 

 Silurian Trilobites: a, Sphcerexochus mirus (Bey.); b, Staurocephalus 

 murchisoni (Barr.); c, Deiphon forbesi (Barr.); d, Calymene niagarensis 

 (Hall); e, Cyphaspis christyi (Hall). (From Chamberlin and Salisbury's 

 "Geology," courtesy of Henry Holt and Company.) 



grew in groves, as it were." 1 About 400 species are known from 

 the Silurian of North America. 



Aster ozoans (Star-fishes) and Echinoids (Sea-urchins) became 

 more common, though by no means abundant. Modern Sea-urchins 

 have exactly twenty rows of calcareous plates tightly fitted to- 

 gether, while Paleozoic forms had a variable number of plates, and 

 in some forms the plates were only loosely joined together, this 

 latter feature apparently being a primitive characteristic. 



Molluscoids. — Bryozoans were less prominent than in the 

 Ordovician, but, nevertheless, they were often common as reef 



1 Chamberlin and Salisbury: Geology, Vol. 2, p. 400. 



