192 C. R. Keyes — Echinodermata of Mississippi Basin. 



tion of vitality. It would seem then that a general amplifica- 

 tion of the various anatomical structures in any faunal group 

 marks its culmination ; and this fact is peculiarly significant in 

 its application to fossil crinoids, for the middle of the Lower 

 Carboniferous was preeminently the climax of crinoidal life. 



The abrupt extinction of a large proportion of crinoidal 

 forms towards the close of the Keokuk is certainly suggestive 

 of a series of decided and wide-spread changes in the geo- 

 graphic and bathymetric extent of the great interior sea. 

 White* has already shown that at least in some portions of the 

 Mississippi basin there were very considerable alterations in 

 the coastal contour of this broad shallow gulf, during the latter 

 part of the Lower Carboniferous ; and it is known that there 

 were even greater changes in the coast line in other parts of 

 this region during the same period. During the Keokuk the 

 waters over portions of Iowa, Illinois, Missouri and Indiana 

 became greatly diminished in depth and the land of the same 

 area was considerably extended. While the St. Louis beds 

 were being deposited the sea again encroached upon the land, 

 extending in some places more than two hundred miles north- 

 ward beyond the former Keokuk waters. Over an extensive 

 portion of the interior sea the conditions of environment dur- 

 ing the Keokuk and St. Louis epoch presented some notable 

 differences, as is amply attested by even a casual comparison of 

 the faunae of the two divisions. In some parts of Indiana and 

 Illinois the organic remains of the former are characterized by 

 a certain luxuriancy of individual growth, apparent not only 

 in one, but in the majority of the zoological groups repre- 

 sented. In the latter division the animal forms, in many 

 cases, are strikingly depauperate. It is not only a depaupera- 

 tion among a few types that is discernible, but a great diminu- 

 tion in vitality is manifest in forms genetically related to those 

 occurring in the Keokuk. 



The Lower Carboniferous of the Appalachian area seems to 

 present two easily determinable divisions. In the Mississippi 

 basin this double nature of the Lower Carboniferous does not 

 appear to be clearly defined. The combined physical and 

 paleontological evidence would indicate that the Lower Car- 

 boniferous throughout the interior of North America is broadly 

 divisible into three sections ; the lower comprising chiefly 

 shales and sandstones, and the two upper principally lime- 

 stones. If, in the correlation of the Lower Carboniferous 

 strata of the Appalachian and Mississippi regions, the latter is 

 to be considered double as the former it does not appear 

 advisable to draw the line of division between the Burlington 

 and Keokuk, for reasons already stated. In accordance, there- 

 * Geology Iowa, vol. i, p. 225, et seq. 



