278 STUART WELLER 
the Salem limestone of a fauna which has some features in common 
with this earlier fauna of a similar earlier odlitic 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 les unconform- 
ably upon the Salem, this unconformity being well shown near War- 
saw, Ill. 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, IIl., 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 odlitic beds, and with the recurrence of 
these conditions favorable for the formation of odlitic 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, IIl., 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 
