396 



SILURIAN PERIOD 



Mid-Silurian fauna. As the sea slowly overspread the continent 

 toward the middle of the period, the increasing room and more con- 

 genial conditions for most forms of shallow-water life resulted in an 

 expansional evolution which produced the Niagara fauna. The 

 families and classes were much the same as in the Ordovician period, 

 but most of the genera were new, and nearly all the species. In 

 general there was a biological advance, though this was not true of 

 all classes. Only the more conspicuous features of the changes will 

 be noted. 



A distinguishing feature of the Silurian fauna was the rich and 

 varied development of the echinoderms, especially the crinoids (Fig. 

 349). They attained such abundance in certain localities that their 

 fragments form the larger part of the limestone. These spots were 

 veritable "flower-beds" of "stone lilies," where beautiful and varied 

 forms grew in groves, as it were. Cystoids were still abundant, and 

 blastoids appear for the first time. Starfishes, serpent-stars and 

 echinoids were unimportant elements in the fauna. 



Fig. 350. SILURIAN BRACHIOPODS: a, Pentamerus oblongus Murch., lateral 

 view of an interior cast; b, Trimerella acuminata Bill., the interior view of a pedicle 

 or ventral valve, showing the elevated platform for muscular attachment excavated 

 beneath; c, Spirifer niagarensis (Con.), exterior view of the brachial or dorsal valve, 

 with the cardinal area and beak of the pedicle valve showing above; d, Chonetes 

 cornutus (Hall), exterior view of ventral valve, showing the cardinal spines; e, 

 Trimerella ohioensis Meek, the internal cast of a highly differentiated inarticulate 

 brachiopod, showing the nngerlike casts of the excavations beneath the elevated 

 muscular platforms; /, Stropheodonta profunda Hall, interior of the ventral valve, 

 showing muscular impressions; g, Spirifer radiatus Sow.; h, Streptis grayi (Dav.), 

 exterior view of the brachial valve, showing the cardinal area and beak of the 

 opposite valve, and the peculiar twisted form of the shell. 



