THE SILURIAN PERIOD 



117 



genera, Spirifer and Pentamerus, made their first appearance and 

 were especially prominent in the Silurian, but became even more 

 so in the Devonian. The Spirifer developed a long, straight, hinge 

 line, while the Pentamerus had a sort of hook-shaped beak pro- 

 jecting over the hinge line. 



Mollusks. — The Pelecy- 

 pods and Gastropods were still 

 common, but they were in no 

 important way different from 

 those of the preceding period. 



Cephalopods were repre- 

 sented only by the Nautiloids. 

 Of these, the straight (Orthoc- 

 eras) forms were still com- 

 mon, but the curved and 

 coiled forms became predom- 

 inant. Some of the Nautiloids 

 were notably ornamented ex- 

 ternally. Otherwise, except 

 for many genera and species 

 changes, the Cephalopods 

 were much like those of the 

 Ordovician, which we rather 

 fully discussed. 



Arthropods. — Crustaceans 

 were represented by Mero- 

 stomes, Trilobites and Eucru- 

 staceans. Horse-shoe Crabs, 

 representing the Merostomes, 

 first appeared in the Silurian. 

 Trilobites culminated in the 

 preceding period, but they 

 still continued to be common^A 



A few new genera appeared^but more disappeared. Silurian Tri- 

 lobites were perhaps more diversified than in any other period. 

 "Like the decadent nations revealed to us in human history, 

 they indulged in extravagant and futile eccentricities, ill befitting 

 their approaching overthrow. Odd and highly ornate forms 

 appeared in profusion (Fig. 64b, c), and in most instances the 

 spines, tubercles, and horns which they produced seem to have 



Fig. 66 

 A Silurian Scorpion, Paleophonus cale- 

 donicus, by Hunter after Peach. 

 (From Le Conte's "Geology," per- 

 mission of D. Appleton and Com- 

 pany.) 



