64 



The azygous interradial is a little larger than either of the 

 other interradials, stands nearly upright, but the superior end 

 curves in upon the vault. The surface bears a short spine, 

 and is covered with pustules and nodes in the same manner as 

 the other plates of the calyx. 



This species is distinguished by its form, surface ornamenta- 

 tion and radiately lined articulating facets. 



Found by C. S. Hodgson, in whose honor we have proposed 

 the specific name, in the Burlington group, in Adams county, 

 Illinois, and now in the collection of S. A. Miller. 



Remarks. — We are led, at this time, after an examination of 

 all of the definitions of the various species of Platycrinus de- 

 scribed from the palaeozoic rocks of North America, and after 

 having examined fossils belonging to nearly all the species, 

 to make the following observations, notwithstanding it would 

 seem to be more appropriate under a discussion of the genus 

 itself. Some species have not been illustrated and have been 

 so poorly defined that they cannot be determined, without com- 

 parison with an authentic specimen, just as they could be de- 

 termined, if there had been no definition, at all, and, there- 

 fore, they are not entitled to recognition. There has been no 

 synonymy, where species have been properly described, and, it 

 is quite possible, there has been none in any case. We recognize 

 one hundred and nineteen species of Platycrinus from America. 



The two earliest forms are P. prcematurus and P. Siluricus, 

 from the Niagara Group. They are widely different from each 

 other, and have no resemblance, beyond the generic formula, 

 to any species that followed them. We have no conception of 

 the forms from which they were derived, and are equally in 

 the dark, as to their descendants. If they fall into the life 

 history of Platycrinus and all species resulted from evolution, 

 what a world of forms lived in the Silurian and Devonian ages 

 that are yet wholly unknown. 



From the Middle Devonian three species have been described, 

 P. eboraceus, P. ericnsis, and P. leal. The first two are quite 

 peculiar and the last might seem to be a fitting ancestor to 

 the round, cup, turbinate, or bowl-shaped forms occurring, in 

 the Subcarboniferous of later times. 



The next described species are found in the lower group 

 of the Subcarboniferous and principally from fche Chouteau 

 limestone. Here we find seventeen species, some of 

 them so widely different from all that preceded them 

 and from all that are known in later rocks, that 



