DEVONIAN AND MISSISSIPPI AN FAUNAS 113 



the Salem limestone of a fauna which has some features in common 

 with this earlier fauna of a similar earlier oolitic bed. 



Superjacent to the Salem is the St. Louis limestone which attains 

 a maximum thickness of 250 feet, but in the northern portion of the 

 Mississippian province it is reduced in thickness and lies unconform- 

 ably upon the Salem, this unconformity being well shown near War- 

 saw, 111. This unconformity indicates that the Mississippian sea 

 retreated to the south during late Salem time, and readvanced in early 

 St. Louis time. The retreat did not reach as far as Alton, 111., however, 

 as near that city the succession is perfectly conformable. The lower- 

 most bed of the St. Louis in the north is a conspicuous limestone 

 breccia which may be a northward continuation of a brecciated 

 horizon near the middle of the formation in the region about St. Louis 

 and Alton, but in following the formation to the south this brecciated 

 horizon becomes less conspicuous and disappears. The fauna of the 

 St. Louis is on the whole a meager one, and is quite different from that 

 of the Salem. In some respects it suggests a recurrence of the Osage 

 fauna, although the species are essentially all different, and some 

 forms, of which the coral Lithostrotion canadense is perhaps the most 

 notable, are distinctly new elements in the fauna. 



The St. Louis is followed conformably by the Ste. Genevieve 

 limestone. This formation differs from the St. Louis and resembles 

 the Salem in the presence of oohtic beds, and with the recurrence of 

 these conditions favorable for the formation of oolitic limestone, there 

 is also a recurrence of the Salem fauna. Many species of the Ste. 

 Genevieve are identical with those in the Salem, although the fauna 

 contains species also which are characteristic to it. Among the latter 

 a conspicuous one near Alton and in Monroe County, 111., is Pugnax 

 ottumwa, this species being present to the exclusion of all others in 

 some localities. The abundance of the same species in the Pella beds 

 of Iowa, the highest division of the so-called St. Louis of that state, 

 suggests the correlation of these beds with the Ste. Genevieve rather 

 than with any part of the St. Louis proper. This occurrence in Iowa 

 is in accord with conditions elsewhere which indicate that the Ste. 

 Genevieve was a time of great expansion of the Mississippian sea in 

 all directions. It was at this time only, during the entire Mississippian 

 period, that limestone conditions obtained in the northern part of the 



