REPORT ON THE ASTEROIDEA, 93 
protrudes over the actinostome, which is contracted and nearly closed. Their armature 
consists of a marginal series of about nine short papilliform spinelets on each plate, 
nearly subequal in length, the outermost being slightly smaller, standing upright and 
parallel to one another. The actinal surface of the plate is covered with numerous rather 
broad granuliform papill, truly granules on the outer portion of the plates, but becoming 
longer and more papilliform as they approach the inner end of the mouth-angle, the 
innermost three or four being nearly as long as, and more robust than, the marginal series. 
All are very obtusely tipped. 
The actinal interradial areas are large and well-developed, extending as far as the twelfth 
or thirteenth adambulacral plate. ‘They are occupied by well-defined plates arranged in 
regular series or columns extending from the adambulacral plates to the infero-marginal 
plates. The innermost plate of each series is normally subequal in breadth to the adjacent 
adambulacral plate, and the columns diminish in breadth as they proceed towards the 
margin. Seven plates may be counted in the series on each side of the median interradial 
line. The surface of the plates is covered with closely placed hemispherical granules of 
similar size and character to those on the adambulacral and marginal plates, disposed in 
regular subrectangular groups, each group being marked out by distinct and well-defined 
sutures or channels from those adjacent. The regularity of the arrangement is constant 
and very conspicuous. 
The anal aperture is subcentral, and, though comparatively large, is inconspicuous, as 
there is no modification in the character of the surrounding paxille. 
The madreporiform body is entirely obscured by paxille, a group of five or six in this 
region being larger and slightly more prominent than any of the others. These indicate 
its position, which is rather nearer the margin than midway between the centre and the 
interbrachial are. It is possible that the madreporiform body may be of large size or even 
compound, judging from appearances ; but these remarks are purely conjectural, as the 
fact cannot be decided without damaging the specimen. 
The ambulacral tube-feet are large and conical, with a very small, mamelon-like, conical 
termination. 
Colour in alcohol, a very light drab, with an ashy grey shade of the same on the 
paxillar area. 
Locality.— Station 44 or 45. Off the coast of North America, east of Delaware and 
Maryland. 
Station 44, May 2, 1873. Lat. 37° 25’ 0” N., long. 71° 40’ 0” W. Depth 1700 
fathoms. Blue mud. Bottom temperature 36°°2 Fahr.; surface temperature 56°°5 Fahr. 
Station 45. May 3, 1873. Lat. 38° 34’ 0” N., long. 72° 10’ 0” W. Depth 1240 
fathoms. Blue mud. Bottom temperature 37°-2 Fahr. ; surface temperature 49°°5 Fahr. 
Remarks.—Plutonaster rigidus is distinguished from Plutonaster ambiguus by its 
larger size, by the large marginal plates, by the absence of a prominent tubercle on the 
