20 



ary radial, which is axillary and supports, upon each upper slop- 

 ing side, a single tertiary radial, which arrangement gives to this 

 ray three arm openings to the vault. There are, therefore, eleven 

 arm openings to the vault in this species. 



In each regular interradial area there is one large, flat plate, 

 followed by two narrow, elongated ones that unite with the plates 

 of the vault. The first azygous plate is in line with the first 

 primary radials and about as large as the smaller one. It is fol- 

 lowed by three plates iu the second range, the middle one being 

 the largest. There are four plates in the third range, and one of 

 the middle ones extends up to the azygous orifice and is trun- 

 cated by it. 



The vault is somewhat elevated over the ambulacral orifices 

 and depressed convex centrally. The largest plate is subcentral, 

 on the azygous side, and bears a long, robust spine. Each plate 

 over a junction of the ambulacral canals bears a long spine. 

 Otherwise the plates covering the vault are plane or slightly con- 

 vex. Nine plates abut upon the subcentral, spine-bearing plate, 

 two of which are truncated by the azygous orifice. Four plates 

 only abut upon the minute azygous orifice, which is situated on a 

 bulbous elevation, about oji a level with the summit of the vault, 

 and opens out horizontally. 



This is the first eleven-armed Dorycriniis ever described, and 

 hence it is distinguished, by that character alone, from all other 

 species. The six spinous plates on the vault and plane surface of 

 the other plates will distinguish it again from D. unicornis. It 

 will also be distinguished from that species by the two plates iu 

 the second range, iu each regular interradial area. And again, 

 by the third range of plates, in the azygous area, and, again, by 

 the fact that, in that species, there are numerous small plates 

 surrounding the azygous orifice, while, in this, five plates consti- 

 tute the azygous bulb and foar of these abut upon the orifice. 

 The surface of the plates, too, in the calyx of that species bears 

 short spines or the plates are very tumid, while, in this, the plates 

 are plane with the exception of the angular radial ridges. Dor)]- 

 criniis u)iic()7-)iis has a wide range. We have examined it from 

 various localities, iu Iowa, from Adams county, Illinois, and from 

 Sedalia, Blackwater and other places in Missouri, and the differ- 

 ences shown by the specimens from these distant localities are 

 very slight, and usually consist in the character of the plates, 



