64 AMERICAN FOSSIL BRYOZUA. lBrij,.i73. 



The Lower Helderberg, succeeding the Niagara, is local in its distribu- 

 tion. But its exposures in Albany and Schoharie counties, New York, 

 have yielded a large number of br3^ozoa, which, however, are not 

 favorably preserved for microscopic study. By some writers the 

 Lower Helderberg is considered Devonian. Its bryozoan fauna indi- 

 cates relationship about equally with both Niagara and Upper Helder- 

 berg. 



The Devonian age was ushered in by conditions unfavorable for 

 brj^ozoan life. The Onondaga and Saliferous have yielded no bryozoa. 

 The Upper Helderberg, in a belt extending from eastern New York 

 west into Ontario, has proved rather prolific. The succeeding forma- 

 tion, the Hamilton, is preeminently a bryozoan epoch. In western 

 New York, Ontario, Michigan, Manitoba, and Iowa its deposits are 

 characterized generally by an abundance of bryozoa in a good state of 

 preservation. A coral reef in the ancient sea, now forming the 

 obstruction in the Ohio River at the Falls of the Ohio, afforded a hos- 

 pitable abode for immense numbers of bryozoa. The later forma- 

 tions of the Devonian, best displayed in New York and Pennsylvania, 

 have yielded very few bryozoa. None have yet been described. 



For the early part of the Carboniferous age the name Mississippian 

 seems to be gradually displacing in this country the terms Subcarbon- 

 iferous and Lower Carboniferous, which have been so variousl}^ used 

 that their use now produces ambiguity. The Mississippian series is 

 characterized in North America by limestone formations, mainly 

 located in the Mississippi Valley, though deposits also occur, not so 

 sharply differentiated, in the trough between the Cincinnati anticline 

 and the eastern highland, to which such local names as Waverly, Mar- 

 shall, and Maxville limestone have been applied. Being limestone 

 formations, they are, as might be expected, very prolific of bryozoa, 

 some, however, much less so than others. The earlier formations, the 

 Kinderhook and Burlington, also have yielded but a limited number of 

 brj^ozoa, but this may be due to the comparatively Hmited areas of 

 outcrop and to lack of systematic collecting. 



In the Coal Measures conditions producing brackish or fresh water 

 and marine formations alternated irregularly. Naturally, marine 

 fossils are quite local in occurrence. But among the marine fossils 

 bryozoa hold their place extremely well, though they are much reduced 

 from the opulence of the Mississippian series in kinds and numbers. 



By the close of the Coal Measures the interior sea had been well- 

 nigh effaced, and in the West and Southwest only — Kansas and Texas — 

 do we find Permian deposits of any considerable extent. But few 

 bryozoa are known from these deposits; whether this is due to their 

 absence or to lack of collecting we do not know. 



During later geological times marine deposits in this country are 

 limited to the Atlantic and Gulf Coastal Plain, to a few favored areas 



