94 



HISTORICAL GEOLOGY 



Molluscoids. — Bryozoans were abundant often as reef builders, 

 particularly in the later portion of the period. Hundreds of Ordo- 

 vician species are known. Though structurally (organically) very 

 closely related to the Brachiopods, they are far different from 

 them in outward appearance, while they look so much like the 



Corals as often to be dis- 

 tinguished from them with 

 difficulty (Fig. 51). The 

 Bryozoans afford a fine 

 illustration of a class of 

 creatures whose genera 

 have changed very little 

 from very ancient times 

 to the present day. 



Brachiopods became 

 much more abundant, 

 more varied, and more 

 complex than in the Cam- 

 brian (Fig. 52). Those 

 with hinged shells (Artic- 

 ulates) greatly outnum- 

 bered the Inarticulates 

 for the first time. Also 

 the shells usually were 

 thicker and more difficult 

 for their enemies to open 

 because of long-hinged 

 lines, or a fluted or ribbed 

 structure, or both. As 

 for the early Paleozoic in 

 general, nearly all were 

 straight-hinged. Many genera and species are known, certain of 

 them having been much used in subdividing the Ordovician sys- 

 tem. Along with the Trilobites, the Brachiopods were the most 

 prominent known organisms of the period. About 300 species are 

 known from the Middle Ordovician of North America alone. 



Mollusks. — As compared with the Cambrian, a wonderful 

 development of Mollusks, both as regards numbers of individuals 

 and species, took place in the Ordovician. 



Pelecypod bivalves were more abundant, usually larger, and of 



Fig. 51 

 Various Ordovician Bryozoans on a slab of 

 limestone. (After R. S. Bassler, U. S. 

 National Museum.) 



