310 New York State Museum 



Chazyan (Lower Ordovician) formations in Appalachian 

 faunas of Atlantic origin. Graptolites are chiefly of un- 

 usual types (with the cups or thecae on one side of the 

 axis, as Monograptus, Cyrtograptus.) They are still 

 common in Europe at this time, but are not often found 

 as fossils in North America. Corals are abundant in the 

 limestones. They formed reefs, which are not common 

 until the Middle Silurian ; but the Silurian coral reefs are 

 seldom of great thickness. Associated with the coral 

 reefs during Middle Silurian (Guelph) time was a large 

 number of species of thick-shelled animals, chiefly gas- 

 tropods, that found it easy to extract the lime occurring 

 abundantly in the warm waters. The honeycomb coral 

 Favosites makes practically its first appearance with 

 many species. An abundance of the chain coral Haly- 

 sites and the tube coral Syringopora is characteristic, 

 though both types appear rarely in Lower and Middle 

 Ordovician formations. Besides these there are many 

 remarkable single corals. Hydro corallines, quite unlike 

 those of the Ordovician Stromatoporas, are abundant and 

 are characteristic types. The bryozoans are numerous 

 in the shaly and pure limestones and many of them show 

 clearly their derivation from Ordovician forms ; but 

 others are of types that appear now for the first time, 

 among them lacelike zoaria of Fenestella, Polypora and 

 Hemitrypa which attained the high points of their de- 

 velopment in the succeeding Devonian and Mississippian 

 periods. Between 300 and 400 brae hio pod forms have been 

 described for North America. This fauna is on the whole 

 very characteristic. Strophomena and Rafinesquina are 

 replaced by Strophonella and Stropheodonta respectively. 

 In the Orthis group are Dalmanella and Rhipidomella. 



