REPORT ON THE ASTEROIDEA, 277 
of the spiniform prolongation of the mouth-plates which is developed in Gnathaster, at 
even an earlier stage than this. It is to be remarked that G'nathaster is much more hispid 
in the young stage than in the adult, a character probably common to other forms of the 
Pentagonasteride. 
7. Pentagonaster arcuatus, n. sp. (Pl. LIL. figs. 1 and 2; Pl. XVIII. figs. 5 and 6). 
Rays five. R=45 mm.; 7=23°5 mm, R=1:937r. The minor radius is thus in the 
proportion of 52°2 per cent. 
General form flat, but moderately thick. Marginal contour stellato-pentagonal, with 
the radial angles produced and tapering to an acute extremity, which is slightly turned 
upward, Interbrachial ares widely rounded. Margins equally rounded abactinally and 
actinally. Abactinal area not elevated above the level of the marginal plates; slight 
depressions are present in the interradial areas near the margin, which are probably 
indicative of a limited capability of inflation. Actinal area subplane, with small well- 
defined depressions external to the mouth-plates. 
The abactinal area is covered with small, subcircular plates, closely placed, united by 
short, narrow prolongations, which leave interspaces for comparatively large papulz in the 
radial regions. The abactinal plates extend to the tip of the ray, two or more series 
separating the outermost supero-marginal plates from the corresponding plates on the 
other side of the ray. Seen from above the abactinal plates have a strikingly paxilliform 
appearance, when their granulation is intact ; the subcircular tabulum is surrounded by a 
marginal, series of small uniform, slightly elongate granules, moderately spaced, and so 
placed that they appear to radiate slightly apart. Within this ring are several small 
hemispherical granules, the majority of which are larger than the marginal series, but are 
in no sense elongate. A small valvate pedicellaria formed by two contingent granules is 
present on the tabulum of a few of the paxille, but these organs are of rare occurrence. 
The plates on the outer part of the ray and those adjacent to the margin throughout are 
devoid of stellate prolongations, and appear to have more or less of an imbricating 
character. 
The supero-marginal plates, which are seventeen or eighteen in number, counting from 
the median interradial line to the extremity, form a well-defined border to the abactinal 
area, which diminishes in breadth towards the extremity of the rays. The plates near the 
interradial line have their breadth rather greater than their length, and the length 
distinctly increases in a few of the succeeding plates, and then diminishes on the outer 
half of the ray. The surface of the plate, which is slightly convex in the transverse 
direction, is covered with rather large, well-spaced, hemispherical granules, those which 
bound the margin being rather smaller than the others, and regularly disposed in lineal 
series. The odd terminal plate is very small. 
