BIGSBY PALEOZOIC ROCKS OF NEW YORK. 361 



Fossils. — A vast concourse of Invertebrates, numbering 180 species 

 in New York, peoples the Niagara group. Among these, 156 new 

 species make their appearance : the orders Monomyaria, Dimyaria, 

 and Gasteropoda are comparatively weak. 



Seven lower Silurian species are found surviving here : — 



Stromatopora concentrica, Phacops limulurus, 



Trematopora tubulosa, Bumastus barriensis, 



Glyptaster brachiatus, Calymene punctata, 



Platyuotus trentcmensis. 



The other seventeen received fossils are from its immediate pre- 

 decessor, and commonly from the calcareous capping. 



The Niagara Group transmits but eleven fossils — four Zoophyta 

 and seven Brachiopoda only, — every group being the scene of renewed 

 creations and renewed destruction. 



A thick bed of Niagara Limestone near the bottom is almost alto- 

 gether made up of crinoid-joints, with some bits of zoophytes and 

 shells, — the higher parts of this low-lying limestone becoming little 

 else than zoophytes partly dissolved, and with an occasional mineral 

 druse. The fossils in the shale below are less injured than here. 



The Crinoidea and Cystidea develope together, both being more 

 numerous in this limestone than either before or after. 



From the Onondaga-Salt group to that of the Devonian Chemung 

 there are not one half as many species as in the Niagara group alone. 

 Thus it is in Europe ; and with zoophytes also. 



The variety in form, appendages, and ornamentation of Crinoidea is 

 much greater in the upper than in the lower stage. They are confined 

 in the Clinton and Niagara to a small thickness of strata ; but they 

 are twice as many as in the lower stage, and the proportion increases 

 upwards. They were gregarious, and are now chiefly found about 

 Lockport. 



A large part of the Lower Silurian corals are even of different 

 genera from those of the upper stage. 



The Brachiopoda are numerous both in species and individuals. 

 Lingula (1), Leptcena (5), and Orthis (7) are fewer, while Atrypa 

 and Spirifer have become more plentiful. Of the six species of 

 Monomyaria, only one, Avicula emacerata, is common. 



Of the five Gasteropoda, Acroculia (Capulus) niagarensis first 

 appears here. 



The Cephalopoda are few in the higher parts of the Niagara group, 

 but abound in the lower. 



The Crustacea are very important. They characterize the lower 

 shaly beds all over New York ; but further westward, limestone, as 

 we know, predominates, with the effect of diminishing the number 

 of trilobites very much, and of proportionately increasing that of 

 zoophyta. The twelve trilobites belong to ten genera, — a fact sug- 

 gesting that many species yet remain to be discovered. 



There is not a single species of the genus Cyathophyllum below 

 the Onondaga Limestone, although representatives of this type came 

 into existence at very early periods ; still no Cyathophyllum ceratites 

 of Goldfuss is known in the Silurian of America, or it is limited to 



