LIFE 



397 



tBrachiopods (Fig. 350) lived on from the Ordovician with no loss 

 if prestige, though most species and many genera were new. The 

 Silurian bruchiopods showed some notable advances in structure. 

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

 abounding vitality than before; but along with the progressive 



developments there were some retrograde modifications. 

 Among molluskSj cephalopods appear to have remained the most 

 powerful inhabitants of the seas. Straight forms were still com- 

 mon, but curved and coiled ones were more numerous. Their 

 shells were more highly ornamented than before, though still plain 

 in comparison with some of their successors. The apertures of the 

 shells of most Ordovician species were circular or oval, but in the 

 Silurian species many of them were curiously constricted (b, Fig. 

 351), especially among the small curved and coiled species. The 



a 



Fig. 351. SILURIAN MOLLUSKS: a and b are cephalopods. a, Orthoccras 

 aiinulatum Sow., a straight chambered shell with annululions; />, Phragmoceras 

 nestor Hall, lateral view of a curved chambered shell with peculiar constricted aper- 

 ture, c, d, and c are gastropods; c, Loxonema If da Hall; d, Platyostoma it iii Circus is 

 Hall; c , Suhiilitcs ventricocus Hall; /, Pterinea emacerata (Con.), exterior view of 

 ft valve of a pelecypod. 



>nstriction appears to have been a protective device. Gastropods, 

 lirly well represented in the Cambrian period and amply in the 

 )rdovician, did not increase greatly in the Silurian. They show 

 idvance in the preponderance of elevated spires, in increased variety 

 of form, and some of them in greater size; but the older types wt-re 

 still plentiful. Pelecypods (/, Fig. 351) were not more plentiful than 

 the Ordovician. 



