148 CRINOIDS. 



and space. The expansion of the several families is also fre- 

 quently indicated by the relatively rapid development, in the 

 supra-generic groups, of certain structural features which soon 

 become curiously differentiated. Perhaps nowhere in any 

 zoological group is its culmination better or more clearly de- 

 fined, in accordance with the suggestions already made, than 

 in the Crinoidea. The remarkable multiplicity of specific and 

 generic types appearing in rapid succession during the middle 

 Lower Carboniferous ; the extreme and phenomenal speciali- 

 zation of particular anatomical structures; the great increase 

 in size, the ponderous character of the test, and the marked 

 structural changes in many minor particulars, are all of pecu- 

 liar biological significance. Toward the close of the Keokukj 

 nearly all of the specialized forms became extinct, and, with a 

 few exceptions, only the more generalized types continued 

 through the Lower Carboniferous — only such forme as were 

 ordinally related to the living crinoids. 



There is one family of the feather-stars, the Actinocrin- 

 idse, the most characteristic section of the group, that illus- 

 trates admirably the genetic relationships of the several generic 

 types. In the American rocks the variety and number of these 

 forms is indeed remarkable — perhaps nowhere equaled in any 

 other age or region. As regards the distribution of the group 

 in time and space, and the phylogenetic history of the camerate 

 forms in general, many pregnant suggestions have been offered 

 recently by certain observations made in the Mississippi valley. 



More than three-fourths of the total number of the genera 

 of the Actinocrinidpe are represented in America, distributed 

 in time as shown in the accompanying chart (plate xi) — the 

 relative expansion of each genus being also indicated. As com- 

 pared with the ages preceding, the lower Carboniferous is here 

 greatly exaggerated in order to show more clearly the relation- 

 ships of the several zoological groups ; for it was during this 

 time that the greatest diversity of form, structure and general 

 ornamentation occurred ; in fact, it was the culmination of crin- 

 oidal life in America. Continuous lines are drawn where the 

 record is complete and the transitions fully shown; while the 



