150 ORINOIDS. 



Before passing, however, to morphological details, it may 

 be well to call attention to some wide-spread variations 

 recorded. Briefly summing, then, the statements recently 

 made in a general consideration of the most marked anatomi- 

 cal features displayed by the Carboniferous crinoids of the 

 Mississippi basin, it may be said that these organisms, from the 

 beginning of the lower Carboniferous to the close of the Keo- 

 kuk, showed: (1) a wonderful and extremely varied develop- 

 ment of the different structural characters; (2) a constant 

 increase in size and massiveness of test ; ( 3 ) a peculiar change 

 in ornamentation, which, from the delicate style of the earlier 

 forms, gradually grew more and more bold and rugged ; and 

 (4) many curious modifications in minor particulars. 



These striking and wide-spread phenomena point to very 

 decided changes in surroundings, such as might have resulted 

 from a gradual decrease in the depth of the sea, a slight diminu- 

 tion in the density of the water, and the introduction of tine 

 sediment in consequence of the nearer proximity to the drain- 

 age courses of the growing continent, or from marked altera- 

 tions in the coastal contour of the neighboring mainland. 

 There probably were acting also numerous other though less 

 apparent influences. Indeed, these suggestions find substan- 

 tiation in the stratigraphy of the region, which gives every 

 reason to believe that the changes went on quietly, though at 

 a rather rapid rate. The great abundance of individuals at 

 this time may be due, in part at least, to the withdrawal of their 

 more motile enemies because of the unsuitable physical impo- 

 sitions already mentioned. The comparatively rapid changes 

 of environment thus imposed would force rapid modifications 

 in the structure of the various iadividuals, in order to secure 

 a more perfect adaptation to the new conditions. And when 

 these physical changes went on with still greater rapidity, 

 structural adjustment was unable to keep apace, and soon 

 ended in the extinction of the group. The unfavorable condi- 

 tions at a somewhat later period are further shown in the 

 neighboring districts, where a few types still persisted, small, 

 depauperate and few in numbers. 



