64 



BULLETIN 75, UNITED STATES NATIONAL MUSEUM. 



basal under arm plates is very variable, within certain limits; the 

 oral shield is usually more or less pentagonal and longer than wide, 

 but it may be rounded triangular or even oval, and it is often wider 

 than long; the second under arm plate is usually squarish but it 

 may be much longer than wide or often wider than long; the number 

 of basal under arm plates which are in contact with each other varies 

 greatly with age. The arm comb, the upper arm plates, and the 

 radial shields do not show very much diversity, but the disk scales 

 are variable and the distinctness of the primary plates differs greatly 

 in different individuals. 



OPHIURA CLASTA, new species.i 



Disk 15 mm. in diameter; arms about 40 mm. long. Disk cov- 

 eretl with a thick skin, in or upon which are numerous more or less 



circular plates of variable size ; 

 these plates do not overlap, 

 and in most cases are scarcely 

 in contact wdth each other; 

 one at the margin of each in- 

 terradius is very much larger 

 than the rest and nearly or 

 quite fills the space between 

 two adjoining pairs of radial 

 shields. Latter, rounded, 

 about as wide as long, more 

 or less lightly in contact, at 

 least distally. First, and 

 sometimes second, upper arm 

 plate more or less tetragonal; 

 remainder small, triangular, 

 and widely separated from 

 each other; all are more or 



FIG. 16.-0PHIURA CLASTA. X3.5. a, FEOM ABOVE; b, j^ss coiupletely brokcn up 

 FROM below; c, side view of two ARM JOINTS NEAR luto Irrcgular fragmcuts, so 



that in many cases the orig- 

 inal form of the plate can scarcely be distinguished. Interbracliial 

 spaces below covered by the huge oral shield, an accessory shield, the 

 two genital plates, and about five marginal plates. Oral shields rounded 

 triangular or pentagonal, about as wide as long. Adoral plates vari- 

 able in shape according to the position of the oral tentacle pore; when 

 this is far back the adoral plate appears to be short and wide (as in 

 the figure), but when the pore is a little nearer the mouth the adoral 

 plate is long and narrow; oral plates well developed and quite dis- 

 tinct. Oral papillae squarish, crowded, five or six on each side, 



« KXaa-cdc, signifying broken in pieces, in reference to the condition of the upper 

 arm plates. 



