NORTH PACIFIC OPHIURANS IN NATIONAL MUSEUM CLARK. 



81 



mm. high at base. Upper arm plates tetragonal with convex distal 

 margin and slightly flaring sides, becoming pentagonal near middle 

 of arm, broadly in contact throughout ; basal plates much wider than 

 long, but the length rapidly increases until the plates are much longer 

 than wide. Interbrachial spaces below covered by oral shield and 

 nine or ten marginal and submarginal plates. Oral shields more or 

 less triangular, outer angles slightly rounded, lateral margins more or 

 less strongly notched at head of genital slits. Adoral plates narrow, 

 diverging; oral plates nearly as large, approximately parallel. Oral 

 papillae stout, more or less pointed, five or six on a side. Genital slits 

 short and inconspicuous. Genital scales short and only little visible 

 from below, but exceptionally 

 wide and conspicuous from 

 above. Each scale carries a 

 marginal series of fifteen to 

 twenty flat, truncate, ap- 

 pressed papillae (see fig. 24), 

 which at upper end of scale 

 are less crowded, and are 

 pointed or rounded at tip, 

 and at lower end pass grad- 

 ually into a series of small 

 papillae on margin of an in- 

 conspicuous, accessory, or 

 supplementary genital scale. 

 Basal under arm plate some- 

 what hexagonal, much wider 

 than long, distal margin longer 

 than proximal; five or six 

 succeeding plates tetragonal, 

 wider than long; remaining 



plates Somewhat hexagonal, FIG. 24. OPHIUEA HADRA. Xl.75. a, FROM ABOVE; b, 



with narrow proximal end, FKOM BELOW; c ' SIDE VIEW * BASE OF ARM - 

 longer than wide, gradually becoming tetragonal, or sometimes dia- 

 mond shaped, and wider than long; all under arm plates in contact 

 with each other until about eighteenth joint, after which they rapidly 

 become widely separated. Side arm plates high but narrow, not 

 meeting above or below on basal half of arm; each plate carries five 

 very small, thick, blunt, well-spaced arm spines; there is no develop- 

 ment of a secondary series as in ponderosa. Oral tentacle pores large, 

 nearly but not quite opening into mouth slit, protected by three to 

 six scales on the outer side and two to four on inner; outermost scale 

 in each series very broad ; tentacle pores of arm very large, the basal 

 ones protected by eight to five scales on proximal side and five to two 

 34916 Bull. 7511 6 



