14 



not ns large, genorally. 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 tlu^ Keokuk (i roup possess ten. twelve, thirteen, 

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

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

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

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

 structed iii)()U a somewhat similar 2)lan. hut others have a deeply con- 

 cave calyx and highly convex >-aidt. None of the Keokuk forms are 

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

 Burlington species, though geniTally tliey are more robust and huve 

 deeper calices and higher vaults. 



The genus Andriroci'inus has not be^^n found, so far as we are ad- 

 vised, in the Waverly (iroup of ( )hio. in the Jlarsliall (i roup of Mich- 

 igan, in the Kinderhook (iroup of Indiana. Illinois or Towa. notwith- 

 standing there are nuTUcrous very fossilift'rous localitit's, 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 

 oidy ill the Chouteau limestone of Missouri. The genus has been 

 recognized, however, at almost ev(>ry locality where the Burlington 

 or Keokuk Croup has been determined, in Iowa. Missouri. Illinois. 

 Indiana, Ohio, Kentucky and Tennessee. The abrujjt ajijiearance of 

 the genns AijaricocrivKs in the Chouteau limestone, in a single local- 

 ity in Missouri, anil its confinement to that geographical locality 

 throughout that geological age. and its distribution over seven states 

 during the two succtH'ding geological ages, that are represented by 

 the Burlington and Keokuk (irouj)s. 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 sjieaking of llic distribution of 

 living giMiera. said : 



"Dr. Bachman pointed out to me ten genera of Ijirds and ten of 

 quadrupeds, all peculiar to North .\merica. but I'ach represi'nted 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 (luadrui)ed originated in one spot only, may 

 explain in a satisfactory manner one ])art of tliis [)henomenon. for we 

 may assume that a lofty chain of mountains opjjosed a jjowerful l)ar- 



