44 BULLETIN -6, UNI-LED STATES NATIONAL MUSEUM. 



rii.liiil nrea; five or six (sometimes four) about each i)late irref^ularly, on inter- 

 radial rofrioi'is and aloiijj border of paxillar area of ray. Abaetiiial plates slightly 

 and irretrularly lobed on papular areas, much less obviously so than in pacijicus. 



Suireromarginai jjlates, forty-one in number from interradial line to extremity of 

 ray, arc much smaller tlian in either of the two other species described below, and 

 have the form of large paxillte, being not conspicuously wider in interbrachial angle 

 than midway along ray. They are irregular or subcircular in outline, rather straight 

 sided adjacent to inferior series, have the appearance of being obliquely oriented, 

 and do not correspond exactly to inferomarginals. Opposite the first ten supero- 

 marginals are seventeen abactinal paxilhc (or irregular transverse series). The 

 supcromarginals are confined to the abactinal surface and their spinelets are heavier 

 and slightly more numerous than on the other jjaxilla'. Terminal j)late wider than 

 long, with a rounded end; notched adjacent to paxillse. 



Inferomarginals much shorter than wide, obliquely oriented (about 45°) to 

 transverse axis of ray, and occupying entire side wall of ray. Each plate is therefore 

 strongly arched, this arch giving the rounded margin. Each plate is covered with 

 spinelets, stouter than those on supcromarginals, somewhat squamiform on exposed 

 surface, and verj' slender in the fasciolar grooves. These fasciolar grooves are deep, 

 and about twice as wide at base of ray (taking transverse axis as length of plate) 

 as the adjacent specialized ridge of the phite. The latter is a thin, almost laminar 

 ridge, very much thinner than in pacijicus, where the specialized ridge is as wide or 

 wider than the grooves. Farther along ray in arcticus the ridges become thicker, 

 nearly or quite as wide as grooves and the inferomarginals are more massive. A 

 longitudinal shallow furrow separates the two series of plates. 



Adambulacral plates with a curvetl furrow series of three or four rather long, 

 slender, blunt, terete spinelets. Lateralmost spinelets slightl}^ the shortest. On 

 actinal surface of plate the spinelets decrease gradually in size outward, there being 

 two or three longitudinal series of three or four spinelets, occasionally more. Some- 

 times a very few odd spinelets stand out of line. The first plate is wider than the 

 rest, with more numerous spinelets. 



Mouth i)lates narrow, the combined pair forming a salient angle into actino- 

 stome. Furrow margin long, with about ten spinules like those of adambulacral 

 plates, decreasing in size outward, and continued along adambulacral margin in 

 several smaller spinelets; innermost two spinules forming "teeth" at angle. An 

 irregular series of spinelets smaller than furrow spinules stantl along edge of median 

 suture; and at outer end of plate are a few intermediate spinelets between the two 

 series, which throughout the longtli of plate stand rather close together. 



Actinal interradial areas fairly large, the plates being arranged in series between 

 inferomarginals and adambulacrals. A single series of plates extends about half 

 length of ray (from interradial angle); a second series, one-fourth; a third series, 

 one-eighth or less. Between first su{)er()marginal ami first adambulacral are about 

 six plates in an interradial direction. 



Madreporic body nearer margin than center of disk; surrounded by sLx or seven 

 large paxillse which overhang the edges; ridges coarse, transverse rather than 

 centrifugal. 



