PTEROPODA. 



193 



In 1873 Hall and Whitfield described Hyolithes gibbosus (probably a variety 

 of H. primordialis), from the Potsdam sandstone. 



In 1876 the writer designated and illustrated Hyolithes principalis, from the 

 Upper Helderberg group, H. aclis and H. striatus, from the Hamilton group. 



In 1877 Dr. T. T. Barrett described H. centennialis, from the Lower 

 Helderberg group. 



Two other species, H triliratus and H. singulus are added to the list in the 

 present volume. 



The American species of the genus Hy^olithes, according to their respective 

 authors, are distributed as follows : 



In tb is enumeration of eighteen species some allowance may, probably, be 

 necessary for repetition, owing to the studies having been made from incom- 

 plete or insufficient material, and from the want of a comparison of collections 

 from the several localities. As these are now recorded, we have nine species 

 from the primordial zone, one species from the second horizon, one from the 

 third ; six species occur in the Upper Helderberg and Hamilton groups which, 

 from the occurrence of many identical species of fossils, are regarded as 

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