The Fauna of the Magnesian Series. 93 



Or to those of th^ '* Potsdam : " 



4. Jordan (Madison) sandstone. 



5. Saint Lawrence (Mendota) formation. 



6. **Dresbach" or "Saint Croix" sandstone and 



the undivided series beneath it. 

 Only numbers 1 to 5, i. e., the Magnesian series (Hall and 

 Sardeson), are fully included in this discussion. 



Of these five formations it is found that the Shakopee 

 has a fauna consisting of species of mollusca,allof which are 

 peculiar to that formation. The New Richmond sandstone 

 has so far yielded no fossils. The Oneota, Jordan and Saint 

 Lawrence, on the contrary, are not only fossiliferous, but the 

 species of each are in part the same as those of the other two. 

 The fauna oi the Oneota dolomite consists mainly of Gaster- 

 opoda, Cephalopoda, a few brachiopoda, and no others 

 except one fragment of a trilobite {Asaphus). The Jordan 

 fauna resembles that of the Oneota, but embraces also trilo- 

 bites like the Saint Lawrence. The last named has yielded 

 but one molluscan species as yet, and several Brachiopoda 

 besides Trilobita. This account may be enlarged in future. 



Nearly all the Trilobita are omitted here, both because 

 they aid only in comparing the Jordan and Saint Lawrence 

 with each other and with underlying strata, all of which 

 have been heretofore united (** Potsdam "), but also because 

 the species have been described from fragments and rare oc- 

 currences, most of which the author has not 3'et had the op- 

 portunity to verify. These will form a problem for the fu- 

 ture. The Mollusca and Molluscoidea are presented below, 

 and from a study of them it has been concluded that the 

 Shakopee is faunally separate from the Saint Peter sandstone 

 above and to a less degree also, from the Oneota below. 

 The New Richmond sandstone maintains uncertain relations 

 between them. With the Oneota are united the Jordan and 

 Saint Lawrence, which last contains a fauna distinct from 

 the next known fauna below it, i. e., that with Obolella 

 polita Hall, Lingula ampla Owen, Hyolithes primordialis 

 Hall. 



The Mollusca, like the Trilobita, occur as casts. In the 

 Oneota particularly, the casts have sometimes filled with 

 chert and are locally more abundant for that reason. A 



