n 



BULLETIN 75, UNITED STATES NATIONAL MUSEUM. 



range of variability of the species. The smallest specimen has the 

 diameter of the disk rather less than 2 mm. with the primary plates 

 covering nearly all of the upper side; the radial shields and the large 

 interradial marginal plates are very small in comparison. The upper 

 arm plates are nearly circular and much swollen, and the under arm 

 plates are longer than wide, but little swollen. The side arm plates 

 are decidedly swollen, and carry two short, sharp, well-spaced arm 

 spines. The oral shields occupy most of the interbrachial spaces 

 below and the tentacle pores have only one or two tentacle scales each. 

 The largest specimen has the disk 17 mm. in diameter and covered 

 by about 150 scales among which the primary plates are not conspicu- 



FlG. 20.— OpHIURA nodosa; variety from icy cape, ALASKA. X6. a, FROM above; 6, FROM BELOW; 



C, SIDE VIEW OF BASE OF ARM. 



ous. The upper arm plates are less swollen, hexagonal or pentagonal, 

 the basal ones much wider than long ; the basal under arm plates are 

 also wider than long, the first one nearly triangular. The side arm 

 plates are very high and little swollen ; they carry from three to six arm 

 spines, of which two are usually larger than the others and of some- 

 what different shape. The tentacle scales have become very numer- 

 ous and there may be as many as eleven (seven proximal and four 

 distal) on a pore. Even far out on the arm there are three or four 

 scales borne on each side arm plate, at the proximal edge of the pore. 

 The chief variations shown by the difl'erent specimens are found in the 

 number and arrangement of the disk scales, the amount of swelling 



