CONDITIONS OF FOSSILIZATION 283 



The articulate Brachiopoda, the Anthozoa, and the Bryozoa 

 of the New York Hamilton Shales are well preserved; but the 

 Mollusca occur as molds and casts. Trilobites, inarticulate brachi- 

 opods, and ostrocods are well preserved. In the Carboniferous 

 shales of Illinois the Mollusca are well preserved. 



In the Mesozoic sandstones of the West the fossils are casts 

 and molds generally, with the exception of the Ostrea and allied 

 genera. 



The best fossils, as a rule, are found in those limestones which 

 contain more or less argillaceous or siliceous ingredients, as in the 

 Waldron beds of Indiana, the Hamilton layers of New York, and 

 the Lower Carboniferous of Crawfordsville, Ind. 



The Schoharie grit, the Oriskany sandstone, and the Calciferous 

 beds along Lake Champlain give siliceous molds on weathering. 



The Upper Helderberg limestone gives molds and casts better than 

 sandy deposits in general. At Cumberland, Md., however, the 

 brachiopods are perfectly preserved as casts of silica in the Oriskany 

 sandstone. 



In the Galena limestone many of the fossils are preserved as 

 casts composed of galena. The coal-measures show molds coated 

 with pyrite. In the Clinton of Oneida County, N. Y., we find 

 limonite casts. In the Trenton of Wisconsin and Tennessee we 

 find casts of sihca. In the Niagara limestone of western New York 

 we find calcite casts; but on the weathered surfaces they are sili- 

 ceous. In the Schoharie grit we find siliceous molds of the interior 

 of brachiopods. 



In general, calcareous skeletal parts show an unequal persistence 

 as fossils in their original condition. Chitinous skeletons are never 

 preserved in their original condition. 



GENERAL CHARACTER OF INVERTEBRATE SKELETONS 



Chitin is confined to the Arthropoda and a few brachiopods 

 which are made up of alternating layers of phosphate of lime and 

 chitinous material, as in Lingula anatina and in the graptolites. 



Silica is confined to the arenaceous Foraminifera, the Radio- 

 laria, the Silicispongia, and Diatoms. 



Calcareous material is confined to the porcellaneous and vitreous 



