THE DEVONIAN PERIOD 



577 



forms, one of which is illustrated in Fig. 414. The crinoids lost 

 the prominence they had in the Silurian and fell to an insignificant 

 place in the fauna, probably because the physical conditions were 

 unfavorable. For like reasons, probably, the corals were relatively 

 few. The cystoids suffered less but were not abundant. Lower 

 forms were present but not prominent, except that the hydrozoan 



f ; i\ 



Fig. 415. The Oriskany Fauna. Brachiopods: a, Rensselceria ovides 

 (Eaton); a representative of a genus restricted to the Helder- 

 bergian and Oriskany (see Fig. 414, d); b, Hipparionyx proximus Van., 

 one of the most characteristic fossils of the arenaceous Oriskany beds; 

 c, Camarotcechia barrandei (Hall), one of the large rhynchonelloid shells 

 of the Oriskany; d } Spirifer murchisoni Castel, and e, S. arenosus (Con.), 

 two of the most characteristic Oriskany species, the first occurring 

 throughout the fauna, the second mainly in the fauna of the arenaceous 

 beds; /, Stropheodonta magnified Hall, a species which sometimes grew 

 to be four or five inches across. The genus has its great expansion in 

 the Devonian. The figures above are much smaller than the fossils, 

 the largest shells being 4 to 5 inches across. The large size of the Oris- 

 kany brachiopod may be appreciated by comparison with Fig. 414, the 

 brachipods of which are reduced to the same extent as those of this Fig. 



Stromatopora became common and formed thick beds. It is 

 notable that fish remains were almost entirely absent from the 

 Helderberg deposits, although present in the preceding and succeed- 

 ing formations. 



The Oriskany fauna. The Oriskany fauna followed the Helder- 

 berg into the interior apparently by a similar route. Its place of 



