96 



HISTORICAL GEOLOGY 



more modern aspect than before. Typical forms are shown in 

 Fig. 53. Ordovician Pelecypods, like their modern representatives 

 (e.g. Clams and Oysters), appear to have thrived unusually well 



Ordovician Pelecypods: a, Cardiola interrupta (Hall); b, Orthodesma? 

 subcarinatum (Ruedemann) ; c, Ambonychia bellistriata (Hall). 



where muds and sands were being deposited, and they are there- 

 fore much more numerous as fossils in the Upper Ordovician shales 

 and sandstones. One important contrast for the reader to keep in 

 mind is the distribution of the Pelecypod bivalves through geologic 



time as compared with the Brachi- 

 opod bivalves. Brachiopods were 

 very abundant and more varied 

 than Pelecypods in the earlier 

 Paleozoic periods, but they have 

 steadily declined almost to extinc- 

 tion at the present time, while 

 Pelecypods have steadily increased 

 in numbers and variety to recent 

 time. 



Gastropods, which comprise the non-chambered, univalve 

 Mollusks, also deployed to a marked degree in this period and 

 predominated over the Pelecypods. These Gastropods were in no 

 essential manner different (except as to species or genera) from 

 existing forms (e.g. the common Snail), and we have here another 



Fig. 54 



Ordovician Gastropods: a, Mac- 

 lurea logani (Salter) ; b, Ophileta 

 complanata (Vanuxem). 



