THE ORDOVICIAN PERIOD 



529 



In some the length of the hinge was increased, apparently affording 

 a better means of resisting the attempts of their enemies to 

 reach them by sliding or rotating the valves past one another 

 (i and p, Fig. 390), while in others the margins of the valves were 

 notched so that the valves interlocked n. The latter device was 



Fig. 391. Ordovician Bryozoans: ,a, Constellaria polystomella Whitfield; 

 b, Crepipora hemispherica Ulrich; c-d, Stromatopora delicatula (James); 

 d is an enlarged portion of c. e-f, Callopora pulchella Ulrich, / being an 

 enlarged portion of e; g-h, Rhinidictya mutabilis Ulrich, h being an 

 enlarged portion of g. 



usually best developed in the shells of narrow and weak hinge-line, 

 where it was most needed. In addition to these devices for prevent- 

 ing the opening of the shell, there was generally a thickening of the 

 shells, and in many cases a ribbing of the exterior, giving strength 

 without needless weight. All these devices seem to imply that the 

 enemies of the brachiopods had increased in effectiveness, but the 

 abundance of the brachiopods implies that their enemies did not 

 gain the mastery. A comparison of the figures of Ordovician and 



