of the Mississippi Basin. 191 



associated crinoids. During the St. Louis and Chester the 

 blastoids were greatly in the ascendancy. This was due partly 

 to an immense expansion in the order itself, and partly to a 

 further marked numerical decrease of the crinoids, which had 

 already become astonishingly reduced. Though the blastoids, 

 in number of genera, were not as abundant in the latter, as in 

 the former, part of the period, individually they were exceed- 

 ingly numerous. 



Recent investigations show that the divisions of the Lower 

 Carboniferous as generally recognized in the continental interior 

 are not as clearly marked by abrupt changes in faunal and 

 lithological features as current opinion considers, and as has been 

 very strongly urged, particularly by some of the earlier geolo- 

 gists. It is now conceded that some of the present divisions of 

 the Lower Carboniferous as presented throughout the region in 

 question, could be very appropriately united. That several of 

 these divisions are very closely related cannot be questioned. 

 There is conclusive proof,* at least so far as the most charac- 

 teristic faunal group is concerned, that the Burlington and 

 Keokuk are more closely related than the two limestones (upper 

 and lower) of the former division. Investigations instituted 

 among the Gasteropoda and Brachiopoda afford like evidence 

 as to the near relationship of the two formations as usually 

 designated. 



From a comparison of the Crinoidea occurring in the Upper 

 and Lower Burlington, and Keokuk limestones it is quite 

 apparent, as first observed by "Whitef and afterwards more 

 fully discussed by Wachsmuth and Springer;}; that the forms of 

 the three horizons present some marked differences. Those 

 species from the lower Burlington are of small size, delicately 

 constructed and ornamented ; in the upper division of the 

 Burlington the peculiar delicacy pervading the lower bed 

 forms is absent, or has assumed a ruder character ; while in the 

 Keokuk the crinoids are characterized by large size, rough 

 massive' construction, bold rugged ornamentation, and a con- 

 spicuous exaggeration in many structural details. The last 

 consideration is of great interest. For it appears that in gen- 

 eral the exaggeration of various structural features is indica- 

 tive of important biologic changes in that particular zoological 

 group in which such extreme developments take place ; and as 

 has been shown in other orders this extravagant and undue 

 acceleration in growth is relatively very rapid, but of short 

 duration, and usually terminates in the abrupt extinction of 

 the group ; or as in some cases results in a very great diminu- 



* Vide Wachsmuth and Springer, Transition Forms in Crinoids, Proc. Acad. Nat. 

 Sci. Phila., 1878. 



f Jour. Boston Soc. Nat. Hist., vol. vii, pp. 224, 225. 

 % Loc. cit. 



