on the Taconic System. 



117 



In this table we find, of previously recognized trilobites of 

 the primordial fauna, two genera and five species ; of previously- 

 known genera of the second and third faunae, four genera and 

 eight species ; two genera before known in the Potsdam sand- 

 stone and seven species ; and of Agnostus^ which is of the first 

 and second faunas, two species ; and one new genus with nine 

 species. 



These are certainly very curious results ; and a modification 

 of our views is still required to allow four genera and eight spe- 

 cies, (or leaving out AmpJiion) three genera and six species of the 

 trilobites of the second fauna to be associated with two genera 

 and five species of trilobites of the piiraordial fauna, and yet re- 

 gard the rock as of primordial origin. 



The brachiopodous genera, Lingula, Discina, Orthis, Leptcena 

 and Stropliotnena, have a great vertical range, and are known in 

 the Lower and Upper Silurian, and most of them in the Devo- 

 nian ; while Camerella so far as known is a Lower Silurian form 

 of the second fauna (perhaps also in a lower position). 



Of the gasteropoda, Maclurea and Ophileta are restricted to 

 Lower Silurian rocks, but occur mainly in the second fauna. The 

 other genera occur likewise in the second fauna and in the Upper 

 Silurian rocks as well as some of them in Devonian. The same 

 is true of the cephalopoda enumerated. 



Tetradium is known in the second fauna of the Lower Silurian 



