230 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



depauperation of the fauna, but the fauna of the Galena is notably com- 

 posed of the larger and more robust forms, probably because the smaller 

 and more delicate shells have been obliterated by secondary chemical 

 changes in the sediments. 



A study of the fauna of the dolomitic Silurian formations of northern 

 Illinois and southern Wisconsin, in connection with the faunas of con- 

 temporaneous non-magnesian formations elsewhere, affords another op- 

 portunity for similar comparison. In this fauna there are recorded 

 fifty or more species of corals distributed among some twenty genera. 

 All of these genera and many of the species occur elsewhere in non- 

 magnesian formations, many of them in the Ohio Valley. Among the 

 Crinoidea seventy or more species are known, belonging to nearly thirty 

 genera. Most of these genera and many of the species are well repre- 

 sented elsewhere in America in non-magnesian formations of essentially 

 the same age, and other genera, not known outside this dolomitic forma- 

 tion in America, are known from non-magnesian formations in northern 

 Europe. The Cystoidea, Brachiopoda, Mollusca, and Trilobita all tell 

 the same story as the Corals and the Crinoids. All these groups are 

 represented in the fauna by many genera and species; the genera are in 

 almost all cases well represented in non-magnesian formations, and a large 

 majority of the species also are common elsewhere. From the considera- 

 tion of this fauna it seems impossible to postulate that the sea in which 

 it lived was any more saline, shallower, or warmer than the contem- 

 poraneous seas whose life is now preserved in non-magnesian formations, 

 either calcareous or argillaceous. 



It is only in the Guelph formation of the Silurian and in its equiva- 

 lents that we may perhaps detect a faunal element indicative of greater 

 salinity, in the association of the more or less diminutive and delicate- 

 shelled species with the large and thick-shelled brachiopods, Trimerella, 

 Monomerella, and Rhinobolus, and the similarly thick-shelled pelecypods, 

 Megalomus and Goniophora. The suggestion that this assemblage of 

 forms in association with the abundance of reef-building corals indi- 

 cates a more than normally saline sea has been made by Clarke and 

 Euedemann, and I am not ready to dispute the truth of their conclusion, 

 but it must be recognized that this same fauna occurs in non-magnesian 

 sediments in the higher beds of the Gotland limestone of Sweden. 



Other widespread dolomitic formations in the American Paleozoics 

 do not lend themselves so readily to the solution of the problem in hand. 

 The widely distributed dolomitic formations of late Cambrian and early 

 Ordovician age are in general rarely fossiliferous, so that their known 



