65 



no resemblance can be traced to any of them, beyond the 

 generic formula. They are as different from a typical Platy- 

 crinus as the most extreme form of Eretmocrinus is from the 

 type of Batocrinus. But, in the Chouteau, we find, in P. colleAti 

 and P. chouteauensis, the commencement of the discoid forms 

 that became so abundant, in the Burlington Group, and, in P. 

 pettisensis, the commencement of the pentagonal, bowlshaped 

 forms that abounded in the Burlington Group and survived 

 until the Kaskaskia age. The rounded, turbinate and cup- 

 shaped forms, however, prevailed. 



More than twenty distinct, discoid species, of which P. 

 cavus, P. gorbyi, P. occidentalism P. sulcatus, P. discoidens, P. 

 excavatus, P. striaebrachiatus , P. sulcifemis, and P. formosus 

 are extreme representatives, have been described from the 

 Burlington Group, but none from rocks of later age, if we ex- 

 cept P. piimilus, from the Warsaw Group, that has a calyx 

 somewhat discoid in outline. Sixteen species having a pen- 

 tagonal, bowlshaped outline, of which P. sampsoni, P. mis- 

 souriensis, P. ivorlheni and P. hodgsoni are extreme forms, have 

 been described, from the Burlington Group, and only three 

 from the Keokuk, one from the Warsaw and one from the 

 Kaskaskia. The Platycrinus seem to have been in their zenith 

 of development, in the Burlington Group, and to have become 

 extinct, before the close of the subcarboniferous period. 



We may divide Platycrinus into discoid, bowlshaped, pen- 

 tagonal, turbinate, hemispherical and urnshaped forms, for con- 

 venience of identification and description. But to propose 

 generic or subgeneric names for such subdivisions would be 

 on a par with those proposed by some authors for Poterio- 

 crinus, and possess no merit, whatever, in a scientific sense; 

 but, on the contrary, would constitute another stake, driven 

 in the path of progress, for every student and naturalist to 

 stumble over, until some one happened along with the strength 

 and courage to pull it up and throw it away. The variety of 

 forms displayed, in Platycrinus, is an evidence of evolution; 

 but until we have become acquainted with many forms that 

 are now unknown and have more correct and settled opin- 

 ions as to the effect of the environments in shaping the struc- 

 ture of the tests, we can know but little of the life history of 

 the animals. 



—9 



