144 CRINOIDS. 



separated by clayey or sandy seams. Here, lying partly en- 

 closed by the hard limestone, are often myriads of stemmed 

 feather-stars, perfect as on the day when they were entombed— 

 forms of wondrous beauty and rare delicacy, gracefully and 

 intricately intertwined like rich, flowing arabesques, and depict- 

 ing clearly the conditions of their environment at the time 

 when they waved slowly to and fro in the secluded depths of a 

 great interior sea. 



Composed of regular plates, definitely arranged and fre- 

 quently highly ornamented, delicate arms and characteristic 

 stems, these organisms were admirably adapted for recording 

 all the marked changes in the physical conditions of their hab- 

 itat. The testimony of the crinoids, corroborating the strati- 

 graphic evidence, points to a slow and very gradual alteration 

 of the sea-bottom. The long period of quietude over the 

 broad Mississippi basin imposed especially favorable condi- 

 tions of environment for a wide geographic and geologic 

 dispersion of the various species. The great uniformity of 

 these conditions over extended areas is amply attested by the 

 occurrence of identical species in localities as widely separated 

 as eastern Iowa and the Lake Valley mining region of New 

 Mexico; or as central Illinois and the southern prolongation 

 of the Appalachians in Alabama. But notwithstanding the 

 extensive distribution of many species, the large majority of 

 the Paleozoic echinoderms were limited in space and particu- 

 larly in time. Those species, therefore, which experienced a 

 wide dispersion form valuable and reliable criteria for synchro- 

 nizing horizoDS far removed from one another. The equiva- 

 lency, however, of strata of distant provinces can at best be 

 only approximately determined from paleontological data alone. 

 For, as has been suggested by Williams, the biologic sequence 

 in any limited region is not indicative of the genetic succes- 

 sion of the inhabitants, but merely the sequence of occupants 

 within that particular area. The gradual oscillation and change 

 of habitat to which the Carboniferous echinoderms of the 

 Mississippi basin were subjected would tend to make their 

 migrations extend through longer periods of time, and their 



