THE SILURIAN PERIOD 



551 



lar or oval, but in the Silurian species many of them were curiously 

 constricted (6, Fig. 402), especially among the small curved and 

 coiled species. The constriction appears to have been a protective 

 device. 



Although the gastropods were fairly well represented in the 

 Cambrian period, and amply in the Ordovician, they did not increase 

 greatly in the Silurian. They show advance in the preponderance 



Fig. 402. Silurian Mollusks: a and b are cephalopods. a, Orthoceras 

 annulatum Sow., a straight chambered shell with annulations; b, Phrag- 

 moceras ne.slor Hall, lateral view of a curved chambered shell with pecu- 

 liar constricted aperture, c, d, and e are gastropods; c, Loxonema leda 

 Hall; d, Platyostoma niagarensis Hall; e, Subulites ventricocus Hall; 

 /, Pterinea emacerata (Con.), exterior view of left valve of a pelecypod. 



of elevated spires, in increased variety of form, and some of them 

 in greater size; but the older types were still plentiful. 



The pelecypods (/, Fig. 402) were not so well represented in the 

 mid-Silurian beds as in the Ordovician, perhaps because the calca- 

 reous bottoms were less congenial to them. 



Corals. The prominence gained by the corals in suitable situ- 

 ations is one of the notable features of the Silurian fauna. In the 

 Ordovician period, the simple forms predominated over the com- 

 pound. The ratio was now reversed. Among the notable types 

 was the unique chain coral (Halysites, Fig. 403, c), which had 

 appeared in the Ordovician; the honeycomb coral (Favosites, a); 



