LIFE 



395 



iprocal of that of the sea; for as the sea contracted, the land 

 anded, and an expansion of land life should have run hand in 

 and with the restriction of sea life. The record of land life is 



too meager to demonstrate that this was the fact. In so far as the 



climate was arid, it was unfavorable for abundant land life. 



Transition from the Ordovician. Of the shallow- water life of the 



early Silurian there is but 



meager record. The eastern 



shore of the continent was 



then farther east than now, 



and the deposits there are 



buried and inaccessible. 



The western border may 



ve been submerged, but 

 e fauna there is little 

 own. 



In addition to the lessened 

 area available for shallow- 

 water life, the conditions 

 probably were less favorable 

 than before. The increased 

 detritus brought to the sea 

 probably inhibited some 

 forms of life, injured others, 

 and helped but few. Some 

 of the basins and bays were 

 doubtless too fresh and 

 some too salt. These gen- 

 eral considerations may ex- 

 plain the meagerness of the 

 faunas of the early Silurian 

 strata. But conditions were 

 not adverse everywhere. In 

 the Gulf of St. Lawrence, 

 Ordovician species lived on 

 for varying lengths of time, 

 and mingled with Silurian 

 species as they developed, 

 and so recorded the transi- 

 tion. 



Fig. 349. SILURIAN ECHINODERMS: a, 

 Eucalyptocrinus crassus Hall, a complete 

 crinoid, showing roots, stem, and body; 6, 

 Holocystitrs adiapatus Miller, a cystoid with 

 irregularly arranged plates and scattered 

 pores; c, Lecanocrinus macropetalus Hall, an 

 articulate crinoid ;d, Troostocrinns rein^'inlti 

 (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. 



