84 THE VOYAGE OF H.M.S. CHALLENGER. 
Chorological Synopsis of the Species. 
Ocean. Range in Fathoms. | Nature of the Sea-bottom. 
= = —_ | 
Plutonaster abbreviatus : . | N. Atlantic. 1000 | Volcanic mud. 
Plutonaster ambiguus . : ..| S. Pacific. 950 | Green mud. 
Bh ed pars ‘ 6 { Globigerina ooze. 
Plutonaster bifrons . : | N. Atlantic. | 210 to 1360 | | Blue a (1340 fathoms). 
Plutonaster marginatus : . | S. Atlantic. 425 | Voleanic sand. 
Plutonaster notatus . : . | N. Atlantic. 1000 Pteropod ooze. 
| Plutonaster rigidus  . : . | N. Atlantic. 1700 or 1240! | Blue mud. 
Plutonaster rigidus,var. semiarmata | N. Atlantic. | 1340 or 13501 | Blue mud. 
1. Plutonaster bifrons, Wyville Thomson, sp. (Pl. XL. figs. 1-4; Pl. XIII. figs. 9 
and 10). 
Archaster bifrons, Wyville Thomson, 1873, The Depths of the Sea, p. 122, figs. 17 and 74. 
Rays five R=90mm.; r= 21mm. R<4‘5r. Minor radial proportion 23°3 per 
cent. Breadth of a ray near the base, 25 mm. 
Rays elongate, moderately broad at the base, tapering continuously to a finely pointed 
extremity. Interbrachial ares widely rounded. Lateral walls vertical. Abactinal surface 
plane, or slightly inflated. Actinal surface subplane, highest at the mouth-angles, and 
sloping thence to the margin and along the rays. 
The paxillee of the abactinal surface are minute and crowded; and are composed of 
twenty to twenty-five short papilliform spinelets—five or six in the midst of each crown 
being more robust than the rest. 
The marginal plates are large, well-defined, and conspicuous. The supero-marginal 
plates, thirty-three in number from the median interradial line to the extremity, form a 
well-defined border on the abactinal surface. They are about as broad as high, and covered 
with granules, which become subconical in form on the lateral half of the plate. Each 
supero-marginal plate bears a single, moderately long, conical pointed spinelet, which 
stands on the rounded angle of the plate, and is directed outward almost horizontally. 
Sometimes this spinelet may be redoubled on two or three plates near the middle of the 
ray. ‘The sutures between the plates are well-defined. 
The infero-marginal plates correspond to the superior series, and are similar in every 
respect, covered with similar subconical granules, and each with a similar and equal-sized 
spinelet directed horizontally. The supero-marginal spines diminish in size towards the 
interbrachial arc and towards the extremity, and also the infero-marginal spines, but in a 
less degree. The largest spmes are consequently about midway on the ray. 
1 The exact station is not known: an alternative record being given. 
