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Observations upon the Genera Glyptocrinus, Glyptaster, 

 Balanocrinus and Lampterocrinus. 



In the first volume of Palceontology of New York, I pro- 

 posed the genus Glyptocrinus for a lower Silurian species, 

 common in the blue shaly limestone of Cincinnati, and else- 

 where iu the west. This genus is characterized by the pre- 

 sence of five basal or subradial plates, succeeded by five 

 series of radial plates of three each, below the first subdi- 

 vision of the ray. The column at its junction with the body 

 is pentalobate. The genus has been recognized and several 

 other species described by different authors. A careful ex- 

 amination of the basal cavity of some of the specimens 

 gives indication that the last joint of the column may be 

 divided, representing five undeveloped basal plates. 



In the genus Glyptaster, five basal plates and five subra- 

 dials are distinguished, with the rays consisting of three 

 plates each, below the first subdivision. 



The distinction between these genera is therefore as fol- 

 lows : In the first the non-development of true basal plates, 

 while the ray is twice subivided before becoming free, and 

 in the typical species the arms are not again subdivided. 

 In the second, a moderate development of the true basal 

 plates, with subradials above, while the ray is but once sub- 

 divided before becoming free. 



At the time of proposing the genus Glyptaster, I was not 

 acquainted with the structure of Balanocrinus of Troost. 

 This genus I recognized in the Wisconsin Report of 1861, 

 describing the species B. inflatus. Upon further examina- 

 tion, it appears that Balanocrinus of Troost, has the struc- 

 ture of Glyptaster with the exception that the basal plates 

 are much more fully developed, while the rays do not bifur- 

 cate before becoming free. Notwithstanding, therefore, the 

 great similarity and near identity of structure in these forms, 

 there seems good reason for the generic separation. 



