14 



not as large, generally, as those from the Keokuk. The Burlington 

 forms described possess either ten or twelve arms, but Prof. Meek 

 mentioned a specimen with eleven arms which he thought was ab- 

 normal. Those from the Keokuk Group possess ten, twelve, thirteen, 

 fourteen, fifteen, sixteen, seventeen or eighteen arms. The Chouteau 

 forms have a depressed body, the calyx is almost flat and only slightly 

 concave about the column, and the vaults are depressed convex or 

 only moderately elevated. Some of the Burlington forms are con- 

 structed upon a somewhat similar plan, but others have a deeply con- 

 cave calyx and highly convex ^ault. None of the Keokuk forms are 

 like the Chouteau species, but some of them resemble some of the 

 Burlington species, though generally they are more robust and have 

 deeper calices and higher vaults. 



The genus Agaricocriaus has not been found, so far as we are ad- 

 vised, in the Waverly Group of Ohio, in the Marshall Group of Mich- 

 igan, in the Kinderhook Group of Indiana. Illinois or Iowa, notwith- 

 standing there are numerous very fossiliferous localities, many of 

 which, like Rockford. Indiana, and Burlington and Le Grand, Iowa, 

 are famous almost all over the world. It occurs in rocks of that age 

 only in the Chouteau limestone of Missouri. The genus has been 

 recognized, however, at almost every locality where the Burlington 

 or Keokuk Group has been determined, in Iowa, Missouri, Illinois, 

 Indiana, Ohio, Kentucky and Tennessee. The abrupt appearance of 

 the genus Agaricocrinus in the Chouteau limestone, in a single local- 

 ity in Missouri, and its confinement to that geographical locality 

 throughout that geological age, and its distribution over seven states 

 during the two succeeding geological ages, that are represented by 

 the Burlington and Keokuk Groups, and its abrupt disappearance 

 before the close of the Keokuk age, is wholly a mystery to us, and 

 can be in no manner accounted for by our knowledge of the theory 

 of evolution. Sir Charles Lyell. in speaking of the distribution of 

 living genera, said: 



''Dr. Bachman pointed out to me ten genera of birds and ten of 

 quadrupeds, all peculiar to North America, but each represented on 

 the opposite side of the Rocky Mountains by distinct species. The 

 theory of specific centers, or the doctrine that the original stock of 

 each species of bird and quadruped originated in one spot only, may 

 explain in a satisfactory manner one part of this phenomenon, for we 

 may assume that a lofty chain of mountains opposed a powerful bar- 



