"Pseudarchaster discus, n. sp. (Pl. XIX. figs. 1 and 2; Pl. XLII. figs. 3 and 4).
Rays five. R = 30 mm.; r = 12 mm. R = 2.5 r.
Rays rather short, broad at the base and tapering gradually towards the extremity, breadth midway between the centre of the disk and the extremity, 6.5 mm. Interbrachial arcs wide and well rounded.
The paxillae of the abactinal area are roundly hexagonal, tabulate, and closely placed, usually with about a dozen short, roundly truncate, polygonal granules, those occupying the central portion of the tabulum being a shade larger than the rest, whilst some of those at the extreme margin are very small. The primary apical plates (bank and first radials) are distinguishable and bear the largest paxillae on the disk, the other paxillae diminish slightly in size as they approach the margin and proceed along the ray. The paxillae are arranged in regular longitudinal lines parallel to a series which occupies the median dorsal line, and which are rather larger than the rest. A considerable number of intermediate plates (paxillae) are present between the dorso-central plate and the five "basal" plates, the position of these being at about one-third of the distance from the centre to the margin. The madreporiform body is comparatively large, and lies external to, and independent of, the adjacent basal plate, which is little more than half its size.
The marginal plates form a well-rounded lateral wall, the curvature of which is semicircular in section. The supero-marginal plates are twenty-eight to thirty in number from the median interradial line to the extremity. In the interbrachial arc the height of the plates is about twice their length, but on the outer part of the ray the height is relatively less. The breadth of the marginal border, as seen from above, is also somewhat greater in the interbrachial arc than out upon the ray. The supero-marginal plates are covered with a closely packed polygonal granulation. The granules are short and flatly truncate, and are twice as large on the inferior half of the plates as on the superior. No prominent spines are present on the supero-marginal plates.
The infero-marginal plates correspond to the superior series, but extend rather further on the actinal area than the superior plates do on their area. The granulation of the inferior series has a marked tendency to become squamiform, especially on the inferior portion of the plate where it is much smaller. On the portion of the plate adjacent to the superior series are usually one or two granules larger and more prominent than any of the others. In the interbrachial arc three short, tapering, sharply adpressed spines stand at well spaced distances on the median line of the plate; but out upon the ray seldom more than one is present.
The armature of the adambulacral plates consists of a furrow series of rather elongate, delicate spines, six in number, which increase in thickness towards their free extremity, and are roundly truncate. External to these is a longitudinal series of three spines, rather widely spaced, shorter than the furrow spines, but rather more robust and tapering ; and these again ate followed by two series of four spines, which are shorter and more closely placed, and usually regular within the disk, but subject to some confusion on the outer part of the ray.
The first series of actinal intermediate (ventral) plates adjacent to the adambulacral plates present a remarkable arrangement of granules. These stand in three or four regular lines, transverse to the furrow, the two lateral series on each plate being inclined slightly towards the corresponding series on the adjacent plate, and thus form a rough kind of fasciole, if such a term were permissible for the short stumpy granules of which they are composed. About four or five granules are present in each series, and one or sometimes two similar and parallel lines of perpendicular and rather more robust granules occupy the median area of each plate.
The remaining portion of the actinal interradial area is covered with short stumpy polygonal papilliform granules, conically pointed at the tip; and here and there one more elongate and spine-like at well spaced distances apart.
The mouth-plates bear two series of spines: one forming a straight line, and proceeding along the side of the suture uniting the two plates, and the other following the opposite outline of the plate and consequently forming a bent series. The six or seven innermost spines, which fall in the furrow margin of the plate, are as long as the adambulacral spines, the foremost spine of all being larger than the rest ; the three or four remaining spines of this series which follow the line of the plate adjacent to the neighbouring adambulacral plate are quite small and papilliform. The inner series, which runs parallel to the median suture of the mouth-plates, is composed of about eight or nine spines, the outermost of which arc little more than granules, but the size gradually increases until the innermost is about equal in length to the marginal series.
Colour in alcohol, bluish grey over the paxillar area and mottled with the same on the actinal interradial areas and the infero-marginal plates within the interbrachial arcs; the rest of the animal is a dirty ashy white.
Locality.—Station 307. In the Messier Channel, between the western coast of Chili and Wellington Island. January 4, 1876. Lat. 49° 24' 30" S., long. 74° 23' 30" W. Depth 140 fathoms. Blue mud. Surface temperature 53.0° Fahr.
Remarks.—This species is readily distinguished from Pseudarchaster tessellatus and Pseudarchaster intermedius by its shorter rays and larger disk, the latter being also flatter. It is further specialised by the presence of the remarkable post-adambulacral fascioles, which arc not present in the other species. Pseudarchaster discus resembles Pseudarchaster tessellatus in having a well-defined line of sharp spinelets along the median line of the infero-marginal plates, and resembles Pseudarchaster intermedius in having no prominent spine in the outer part of the adambulacral armature."
(Sladen, 1889: 110-112)
Pseudarchaster discus is a species of starfish from the Pseudarchasteridae family. This starfish has a mouth opening on its bottom side. The torso consists of a disk with arms. This starfish can regenerate a lost or damaged arm from its body.
This species belongs to the genus Pseudarchaster and can be found in the Antarctic Ocean and the Magellan Strait. The scientific name of this species was first published in 1889 by Salden [1]
Pseudarchaster discus is a species of starfish from the Pseudarchasteridae family. This starfish has a mouth opening on its bottom side. The torso consists of a disk with arms. This starfish can regenerate a lost or damaged arm from its body.
This species belongs to the genus Pseudarchaster and can be found in the Antarctic Ocean and the Magellan Strait. The scientific name of this species was first published in 1889 by Salden
