29 



elsewhere, by reason of the increased number of radial plates. 

 The arms are composed of a double series of short plates, from 

 the commencement, united by the usual zigzag suture. 



There is s single, long, twelve sided plate, in each regular inter- 

 radial area. It rests between the superior sloping sides of the 

 first primary radials, curves over the summit of the calyx sepa- 

 rating the primary, secondary and tertiary radials, and unites with 

 two plates belonging to the vault. There are four plates in the 

 azygous area. The first one is, in the lower half, in line with the 

 first primary radials, but it is more than twice as long as a pri- 

 mary radial and curves over the calyx so as to be visible, in a 

 side view, as shown in figure 6. It is followed by a short plate 

 that unites with two plates belonging to the vault. There is a 

 plate, on each side of the first and second plate above described, 

 that rests upon one superior sloping side of a first primary ra- 

 dial, is of about the same size as a regular interradial, has nine 

 sides, and curves over and unites with two plates belonging to 

 the vault. Each of these plates abuts one side against the primary, 

 secondary and tertiary plates and the other against the first and 

 second azygous plates. 



The vault is subpyramidal and flattened or slightly concave in 

 the interradial areas. There is a very large, tumid plate at the 

 apex of the vault, which is surrounded by six large, convex plates 

 and one narrow elongated plate, in the azygous area. At least, 

 there is only one plate at the top of the azygous area in the speci- 

 men being described, which may be seen in figure 5. There is a 

 single, very large, tumid plate over the lower part of three of the 

 ambulacral areas, and two over the lower part of the other two 

 areas. These large plates cover the ambulacral canals, where they 

 unite, in each radial series. All other plates in the vault are 

 smaller than those described, but they vary much in size; some 

 are convex, others fiat, and others concave. Those in the inter- 

 radial areas are not numerous and are generally elongated. The 

 azygous area is almost wholly taken up with a remarkable bulb- 

 ous, elliptical prominence that bears the azygous opening. The 

 plates in this area are rather large, generally, and conform to the 

 surface elevations and depressions. 



This sp?cies is distinguished from all others heretofore described 

 by the characters of the regular interradials and azygous plates, 

 by the azygous area and bulbous prominence and by having twelve 

 arms. Many other differences might be pointed out, but we deem 

 these characters amply sufficient. We might begin to compare it 

 with Agaricocrinus profundus, by saying that the calyx is pro- 



