THE SILURIAN PERIOD 



549 



though they suffered an almost entire change of species and a very 

 large change of genera. When it is considered that the brachiopods 

 were among the most resistant and conservative of the inverte- 

 brates,, this large change of species and genera emphasizes the 



Fig. 400. Silurian Echinoderms: a, Eucalyptocrinus crassus Hall, a com- 

 plete crinoid, showing roots, stem, and body; b, Holocystites adiapatus 

 Miller, a cystoid with irregularly arranged plates and scatterd pores; 

 c, Lecanocrinus macropetalus Hall, an articulate crinoid; d, Troostocrinus 

 reinwardtii (Troost), showing the typical bud-like form of a blastoid; 

 e, Caryocrinus ornatus Say, a cystoid with regularly arranged body plates. 

 Pores in radiating lines from centers of plates. 



stress of the conditions that controlled the transition, and its bio- 

 logic importance. The Silurian brachiopods had gained in differ- 

 entiation, and had made some notable advances in structure. On 

 the whole they were more robust and gave more obvious signs of 



