53 



Basal plates form a pentagon less than one-fourth wider than 

 the diameter of the column and having an hemispherical de- 

 pression for the insertion of the end of the column. 

 First primary radial a little wider than long, subequal in size, 

 and the calyx will rest on the points of the central nodes in 

 these plates. Second primary radials large, very little wider 

 than long, quadrangular. Third primary radials about the size 

 of the second, pentagonal, axillary, and in two of the rays sup- 

 port on each upper sloping side a single secondary radial 

 which is axillary and supports on each upper sloping side two 

 tertiary radials which gives to each of these rays four arms. 

 These two rays adjoin the azygous area. In each of the lateral 

 rays the third primary radial supports on one upper sloping 

 side three secondary radials, and on the other a single sec- 

 ondary radial which is axillary and supports upon each upper 

 sloping side two tertiary radials, which gives to each of these 

 rays three arms. In the ray opposite the azygous area the 

 third primary radial supports upon each upper sloping side 

 three secondary radials which gives to this ray two arms. 

 There are, therefore, sixteen arms in this species. The arm 

 formula is 4+4+3+3+2. 



The interradial areas are substantially alike. The first in- 

 terradials are the largest plates in the calyx, have nine sides, 

 and are truncated above for the second interradial, which is 

 rather less than half as large as the first. It is followed in 

 the third range by a small plate that unites with two plates 

 belonging to the vault. 



The vault is convex, elevated over the ambulacral canals 

 and concave in the interambulacral areas, and bears a short 

 subcentral proboscis, at the summit of which is the azygous 

 orifice. The plates are large, covered with granules, and each 

 one bears a central tubercle except the two elongated plates 

 in concave depression in each interambulacral area. There arc 

 two ovarian apertures between each ray, and also between 

 each double ray and the single rays, which arrangement gives 

 to the species twenty ovarian apertures. This species is dis- 

 tinguished by its general form and surface ornamentation from 

 all other sixteen armed species. It is also distinguished from 

 D. ccelaius and all species except D. salebi-osus by the aim 



