18 



There are eight plates in each regular interradial area. The 

 first ones are the largest plates in the calyx, excepting, only 

 one of the first primary radials. They are heptagonal, longer than 

 wide, truncate the siibradials and extend nearly as high as the 

 second j)rimary radials. There are two plates in the second range 

 equal in size, and larger than the third primary radials. These are 

 followed by three much smaller and somewhat elongated plates in 

 the third range, and these, in turn, are followed by one plate in the 

 fourth range, which reaches as high as the tertiary (?) radials 

 above described, and which, we suppose unites with the plates of 

 the vault, as we suppose the single secondary interradials did. 

 No part of the plates of the vault is preserved, in our specimen, 

 neither are any of the arm openings to the vault, and it is, there- 

 fore, proliable that the plates which we have called tertiary radials 

 on tlie outer upper sloping sides of the secondary radials are inter- 

 radials, and, if so, only ten arms arise from the calyx, which are 

 separated by wide interradial areas connecting with the vault. 



The azygous interradial area differs from the regular interradial 

 areas chiefly in the fact that it contains an additional, large plate, 

 in the second range, which occupies the usual position of a second 

 primary radial. Take out this long heptagonal plate and close up 

 the space by straightening the crooked primary radial series and 

 then all the interradial areas will be substantially alike. 



The absence of radial ridges, general form of tiie calyx, number 

 of interradials and peculiar position of the large heptagonal plate 

 in the second range of azygous interradials will distinguish tliis 

 species from all others hitherto described. Walter R. Billings fig- 

 ured and described three plates, in the second range, in the azygous 

 area, of Archa'ocrinus dcsideraftis, but the additional plate is like 

 other plates, in the second range of the interradial areas, and did 

 not disturb the adjacent primary radial series. We do not consider 

 the absence of radial i-idges or peculiar position of the azygous 

 plate as of more than specific importance and hence have no hesi- 

 tation in referring this species to the genus Archa'ocrinus. 



Found in the Trenton Group in Knox County, Tennessee, and 

 now in the collection of Wm. F. E. Gurley. 



