42 



upon each superior sloping* side three secondary radials, which 

 gives to each of these rays two arms. In the ray opposite the 

 azygous area the third primary radial bears upon one upper 

 sloping side a single secondary radial, which is axillary, and 

 supports upon each upper side two tertiary radials, and upon 

 the other three secondary radials, which gives to this ray two 

 arms. There are, therefore, fifteen arms in this species. 



The azygous area is very much like the other interradial 

 areas. The first interradials are large and have nine sides 

 and are truncated above by the second interradials. The sec- 

 ond interradials are about one-third as large as the first, and 

 are followed in the third range by a somewhat smaller plate 

 that unites with two elongated plates belonging to the vault. 



The vault is slightly convex, and correspondingly depressed 

 in the interradial areas toward the margin. It is covered by 

 two circles of plates and a few intercalated ones, the surface 

 of which is beautifully ornamented with coarse granules and 

 delicate nodes. It bears a subcentral azygous orifice slightly 

 elevated above the rest of the vault, and which appears to be 

 about complete in the specimen illustrated. There are small 

 ovarian apertures passing through the plates forming part of 

 the covering of the ambulacral canals near the base of the 

 arms. One on each side of each pair of arms and one on each 

 side of the single arm, which gives to this species sixteen 

 ovarian apertures. 



This species most resembles D. charlesiownensis. In that 

 species the arm formula is 3-|-4-f-3-|-2-f-3, while in this species 

 it is 4-f-2-j-3-j-4 + 2. The arms are so arranged in that species 

 that the ray on each side of the azygous area has three arms, 

 while in this, one has two arms and the other four. The pro- 

 portional size of the plates and the surface ornamentation also 

 differ and there are other minor differences. The arm formu- 

 las, however, are sufficient to readily distinguish the species 

 at all times, and such differences are always of specific im- 

 poi tince. 



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

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

 Gurley. 



