47 



ondary radials, which gives to each of these rays two arms. 

 There are, therefore, sixteen arms and sixteen ambulacral 

 openings to the vault in this species. 



The interradial areas are substantially alike. The area be- 

 tween the two armed rays is slightly more bulged than the other 

 areas, and the first plate is somewhat larger than it is in the 

 others, which indicates that it is the azygous area. The first 

 interradials are the largest plates in the body, have nine sides, 

 and are broadly truncated above for the second interradials. 

 The second interradials are short and wide and the superior 

 part curves in and unites with the plates of the vault. 



The vault is only slightly convex over the central part and 

 radial areas, and is very much depressed toward the margin, 

 in the interradial areas, so that the radial areas stand up and 

 project beyond the margin of the calyx and have the ambula- 

 cral openings directed upward, The plates over the junction 

 of the ambulacral canals bear nodes and the other plates are 

 tubercular, but our specimens show them somewhat eroded 

 and the sutures between the plates can only be, in part, 

 traced, and for those reasons these features are not shown in 

 the illustrations. There are sixteen ovarian apertures, one 

 close by the side of each ambulacral opening to the vault. 



This species would seem to be more nearly related to D. 

 salebrosus, than to any one hitherto described, but the base is 

 depressed, in that species, and truncated and the vault elevat- 

 ed more than in this, so that they may readily be distinguished 

 by the form. The surface ornamentation too is somewhat 

 different in the two species, and so are the interradials, 

 and in that species no ovarian pores have been discovered, 

 ered. But the arm formula alone is sufficient to distinguish 

 them, and to separate this from all other described species. 

 In D. salebrosus, the arm formula is 4+4+3+3+2=16. In 

 this species it is 4+4+4+2+2=16. The azygous area in that 

 species, if it has any, is between the four-armed rays, and in 

 this between the two armed rays. 



Found by Geo. K. Greene, in the Hamilton Group, near 

 Charlestown, Indiana, and now in the collection of Wm. F. E. 

 Gurley. 



