26 



each first radial to a central tubercle on the adjacent first radials; 

 column round, perforation small, cinque-foil. Surface ornamented 

 by rather strong radial ridges commencing at the central node on 

 the first primary radials and extending to the arms, and by radi- 

 ating ridges from a central node on each plate. 



Basal plates sunken and so nearly covered by tlie column as not 

 to be visible externally. First primary radials about as long as 

 wide, one-half the length being in the columnar cavity. Second 

 radials, quadrangular, wider than long, flattened and bearing a 

 prominent central node. Third radials, pentagonal, expanding to 

 the late:-al angles, wider than high, four of them bearing upon 

 each upper sloping side four secondary radials and the other one 

 bearing upon one upper sloping side four secondary radials and 

 upon the other a single secondary radial which bears upon each 

 of the upper sloping sides three tertiary radials. Four of the 

 radial series thus bear two arms each and the other bears three 

 arms, making eleven arms in this species. From the arm bases it 

 might be inferred that the arms are composed of a double series 

 of interlocking plates, as there are two plates at the base instead 

 of one, but two furrows are not seen to enter the vault. 



First iuterradials the larger plates of the calyx and have nine 

 sides. Second iuterradials less than half as large as the first, 

 hexagonal, and support three small plates in the third range, 

 which are followed by three smaller plates that separate the arm 

 bases and unite with the plates of the vault. Intersecondary radi- 

 als two, separating the arm bases and uniting with the plates of 

 the vault. No azygous side. 



Vault moderately convex, with a subcentral proboscis and com- 

 posed of convex polygonal plates. Those in the interradial areas 

 are elongated and disposed in a fan-like arrangement. Four pores 

 or passages enter the vault in each interradial area, and two in 

 each intersecondary area; they are continued by a shallow furrow 

 across the top of the last range of iuterradials. 



This species is more nearly related to D. laois than to any other 

 that has been described. It is distinguished, however, by having 

 eleven arms instead of ten, which, alone, wo regard as of specific 

 inii)ortauce. It is further distinguished by having one more sec- 

 ondary radial and one more interradial in the third range, which 

 we think is of specific importance, especially as our specimen is 

 much smaller than any specimen of D. lacus we have seen. The 



