THE ORDOVICIAN PERIOD 



433 



Bryozoa. — Fossil bryozoans (Greek, bruon, moss, and zoon, animal) 

 consist of small branching stems and lacelike mats (Fig. 404 A-E), the 

 skeletons of minute animals that lived in colonies. They resemble cer- 

 tain corals in external appearance, but are related to the brachiopods. 

 They can, as a rule, easily be distinguished from corals by the smaller 

 size of the colonies in which the polyps lived. Bryozoan fossils are 

 very common in limestones of Ordovician age and were important 

 limestone makers. They are valuable " index fossils " in determining 

 the age of Ordovician strata, since they were abundant not only in 

 individuals but also in species. 



Mollusca 



Pelecypods are abundant in the salt and fresh waters of the present, 

 being represented by the clam, pecten, oyster, and many others. 

 They have bivalve l1ffWfc 



■ . ■ 



B 



A 



shells in which the 

 two valves are usu- 

 ally nearly alike (Fig. 

 405 A-E). In exter- 

 nal form they differ 

 from brachiopods, 

 which they resemble, 

 in the lack of bi- 

 lateral symmetry. 

 Aside from fossils 

 whose relationships 

 are doubtful (Fordilla 

 and Modioloides) this 

 great class is almost 

 unknown previous to 

 the Ordovician. As a rule, pelecypods are rather rare fossils in the 

 Ordovician rocks, being more abundant in sandstone and shales than 

 in limestones, thus showing that they lived best on sandy and muddy 

 bottoms. 



Gastropods. — This class was more abundant than the pelecypods, 

 and even in the early Ordovician was represented by a considerable 

 variety of forms (Fig. 406 A-G) which closely resemble modern rela- 

 tives in external appearance. 



Fig. 405. — Ordovician pelecypods: A, Pterinea de- 

 mis sa; B, Rhytimya radiata; C, Cyrtodonta billingsi; 

 D, Ctenodonta nasuta; E, Byssonychia radiata. 



