(i.S BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



This furrow spine is slenderer than the subambulacrals, tapering, bluntly pointed 

 or obtuse, while the other spines axe heavier, subcylindrical, round-tipped, some- 

 times a trifle clavate or compressed. A considerable number of consecutive plates 

 may have hut one spine, or may have two. The spines bear thick, subterminal 

 clusters of broadly lanceolate, unequal, straight pedicellariae, together with smaller 

 blunl crossed pedicellariae, tho latter being more numerous on the subambulacral 

 than on the furrow spines, where the straight pedicellariae predominate and are of 

 larger size. Some of tho specimens with numerous marginal spines have two and 

 three 'ldnmbulacral spines on a portion of the proximal part of the ray. The third 

 (and outer) spine is shorter than the others. The armature of the furrow margin 

 appears crowded by reason of the more numerous spines and their heavy masses of 

 pedicellariae; station 3274. 



Four pairs of adambulacral plates are contingent behind the mouth plates 

 (fewer in the small specimens). The mouth plates are small, not especially sunken, 

 scarcely wider than the first pair of combined adoral adambulacral plates, and 

 their greatest length equals that of the first two or avo and a half adambulacral?. 

 There is a very short, stumpy, pedicella -ia-clad spine directed across the mouth >f 

 furrow (and reaching about to the middle); another longer spine, tapering and blunt, 

 directed over actinostome, and a third, suboral, still longer, slightly tapering, and 

 -t.uuling erect like the following adambulacral spines. 



The madxeporic body is prominent, convex, situated at about the middle of r, 

 and has irregular radiating striae. 



Papulae numerous in small irregular groups dorsally and irregularly distributed 

 as the meshes of the skeleton; prominent just above, between, and just below 

 marginal plates, and present also between the actinal plates (when there is more than 

 erne series), and adjacent to adambulacrals. 



Straight pedicellariae (pi. 32, figs. 2 to 2e) are usually abundant on all surfaces 

 and on the adambulacral spines and vary considerably in size, the largest usually 

 occurring in the axillary channels. (See variations.) The larger straight pedicellariae 

 of acervaia is a distinguishing feature from polaris. Arctic specimens have propor- 

 tionately more numerous straight and fewer crossed pedicellariae on the adambu- 

 lacral spines than have southern examples. 



Crossed pedicellariae (pi. 30, figs. 3, 4) vary greatly in size in different parts 

 of the range. The abactinal measure from 0.22 to 0.3 mm. in length. The arctic 

 specimens have pedicellariae more closely resembling those of polaris than have 

 the southern Bering Sea forms. For measurements see explanation of plates. 

 Numerous crossed pedicellariae occur between the abactinal spines as well as imme- 

 diately surrounding them. 



Color in life: Above, clouded with very dark brown; madreporic plate cream- 

 colored. Sides of rays and actinal surface, yellowish cream color (Stimpson). 



Anatomical notes. — The cleaned skeleton of the ray consists of a rather close 

 irregular network of very unequal, irregular plates, there being a variable number 

 of large convex, irregularly lobed plates corresponding to the groups of spines. 

 These seem to have no order of occurrence except that the majority are near or on 

 the radial line The intermediate and smaller primary plates are oblong, and of 

 various polyhedral forms with rounded corners. Twelve to fourteen plates can 



