286 



PALEOZOIC TIME — DEVONIAN AGE. 



3. Articulates. — (a.) Crustaceans. — The Trilobites Phacops Bvfo (fig. 

 499) and Dalmania eaUiteles (fig. 500, representing the posterior extremity) are 

 common in the Hamilton beds. 



Figs. 498-500. 



Cephalopod.— Fig. 498, Goniatites Marcellensis. Trilobites. — Fig. 499, Phacops Bufo; 500, 

 Caudal extremity of Dalmania calliteles. 



(6.) No Insects are known, although the land had its vegetation. 



General Observations. 



Geography. — The positions and nature of the Hamilton beds 

 indicate similar geographical conditions to those of many earlier pe- 

 riods, — that a shallow sea covered New York and spread widely to 

 the west, and that many changes were experienced in the water-level ; 

 the beds are to a great extent mud-beds, whence we learn that 

 they were deposited in quiet waters ; the fossils are marine, proving 

 marine waters. The beds in New York are thickest about its cen- 

 tral parts, and yet spread to its eastern and western limits, ex- 

 cepting the Upper, the Genesee shale, which is not known in the 

 eastern part ; they are partly calcareous in the lower part of the 

 Marcellus beds, proving that the change from the condition of the 

 limestone-making Corniferous period was gradual ; limestone layers 

 occur higher up, at intervals, indicating changes of level, which 

 favored at times Encrinites and corals ; ripple-marked flags make 

 up some layers, proving by their evenness and extent, and the 

 regularity of the lamination, that the sea at the time of their forma- 

 tion swept over extensive sand-flats, coming in over the present 

 region of the Hudson River or of New York Bay. The existence 

 of a barrier of sand along the ocean, such as is thrown up and at 

 intervals removed again by the waves, would account for the vary- 



