of the Mississippi Basin. 187 



interstices, with scarcely any finer and cementing materials; 

 while throughout are disseminated broken and shattered calyces, 

 fragments of arms, and portions of stems. Other parts of the 

 formation exhibit thick beds of compact massive limestone 

 alternating with layers of less durable composition. In some 

 of the thin, sandy or clayey partings, lying half imbedded in 

 the surface of hard limestone are often myriads of stemmed 

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

 forms of wondrous beauty and rare delicacy, gracefully and 

 intricately intertwined like some rich flowing arabesque ; and 

 depicting accurately and distinctly the conditions of their sur- 

 roundings when they waved to and fro in the quiet depths 

 of the great Carboniferous 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 marked changes in the physical conditions of their habitat. 

 The testimony of the crinoids, corroborating the stratigraphic 

 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 conditions of 

 environment for a wide geographic and geologic dispersion of 

 the various species. And the great uniformity of these con- 

 ditions over extended areas is amply attested by the occurrence 

 of identical species in localities as widely separated geograph- 

 ically as eastern Iowa and the Lake Valley region of New 

 Mexico ; or as central Illinois and the southern prolongation of 

 the Appalachians in Alabama. But notwithstanding the ex- 

 tensive distribution of many species, the large majority of 

 Paleozoic echinoderms was very limited in space and especially 

 in time. Those species therefore which experienced a wide 

 dispersion form valuable and reliable criteria for synchronizing 

 horizons far removed from one another. The equivalency, 

 however, of strata of distant localities can at best be only ap- 

 proximately determined from paleontological data alone. As 

 has been suggested by Williams,* the biologic sequence in any 

 limited region is not indicative of the genetic succession 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 specific ex- 

 istence more protracted than the stratigraphy of any one place 

 would indicate. And thus certain species would become 

 extinct in one region and be completely replaced by very differ- 

 ent forms ; while in distant localities the migratory species 

 would continue to flourish in all their wonted vigor. 



*Proc. Am. Association Ad. Sci., vol. xxxiv, p. 232, 1885. 



