DIFFERENTIATION AMONG THE CRINOIDS. 201 



senses — sometimes referring to the Burlington alone, sometimes to both 

 Burlington and Keokuk, and often to the Burlington and a part of the 

 Keokuk. Partly on lithological grounds, but chiefly for paleontological 

 reasons, the " Osage " may be regarded as made up of three members — 

 upper, middle and lower — coinciding essentially with the Keokuk and 

 the upper and lower Burlington limestones. In regard to the fossils of 

 the three horizons, the most conspicuous differences are found among 

 the crinoids, which form such a characteristic feature of the several 

 faunas. These general differences were first suggested by White* and 

 quite recently f they have received further attention. They may be 

 restated briefly here: Those species from the lower Burlington are of 

 small size, delicately constructed and highly ornamented. In the upper 

 division of the Burlington the peculiar delicacy pervading the forms of 

 the lower bed is absent or has assumed a ruder character, while in the 

 Keokuk the crinoids are characterized by large size, rough and massive 

 const ruction, bold and rugged ornamentation, and a conspicuous exag- 

 geration in many structural details. The last consideration is of great 

 interest, since it appears that in general the exaggeration of anatomical 

 features is indicative of important biologic changes in that particular 

 zoological group in which such extreme developments take place. 



It is apparent from a close study of the crinoids (and in a somewhat 

 less marked degree among other zoological groups) that there was an 

 abrupt change of physical conditions at the close of the Keokuk epoch. 

 One-half of the Carl »< >niferous genera had become extinct ; the great group 

 Camerata bad passed away, with the exception of the Hexacrinida 1 and 

 a few depauperate forms of several other genera whose existence was 

 quickly brought to a close. A large proportion of the genera in the 

 extensive section Inadunata had disappeared; of those groups which 

 survived to the close of the period, a diminutive species was the sole 

 representative of the Larviformia, while of the great group of the Fistulata 

 only the ty] deal genus (including several subgenera) of the Poteriocrinida' 

 extended through the entire lower Carboniferous. 



As already stated in another place, the sudden extinction of a large 

 proportion of the crinoidal and other forms of life at the close of the 

 Keokuk is certainly suggestive of a series of wide-spread changes in the 

 geographic and bathymetric extent of the great interior sea ; and there 

 is sullicieiit evidence to indicate that at the close of the Keoknk and 

 during the early part of the so-called Warsaw the northern coasl line of 

 the broad shallow gulf moved rapidly southward, and that this movement 

 was soon followed by a slight depression. The St. Louis waters then 

 pushed northward again, in some places several hundred miles. 



* J ii. Boston Soe. Nal II i t., vol. vii, pp. 224, 225 



• ■- Am. Jonm. Sei., 3d series, vol xxxviii 1889 pp 191 L92 



XXXIX B i 3oi \ m \ "i '■ I'M 



