dcsimg

Description

provided by NMNH Antarctic Invertebrates

Bathybiaster loripes, n. sp. (Pl. XXXVI. figs. 1 and 2; Pl. XLII figs. 1 and 2).

Rays five. R= 82 mm., r=14.5 mm. R > 5.5 r. Breadth of a ray near the base (between the third and fourth supero-marginal plates), 15 mm.

Rays moderately elongate, tapering continuously from the base to a finely pointed extremity. Lateral walls high and nearly vertical. Interbrachial arcs acutely rounded. Abactinal surface slightly inflated over the disk, and very slightly convex along the rays. Actinal surface of the rays convex and merging gradually into the lateral wall, which gives the rays a distinctly cylindrical appearance when viewed from below. On the other hand, when seen from above they appear more or less flat in consequence of the small degree of convexity of the abactinal area.

The abactinal surface of the disk and rays is covered with numerous rather small and closely crowded paxillae. These are low and of uniform height throughout, and consist of seven to twelve small, very short, clavate or flaring, skin-covered, papilliform spinelets —one or two irregularly central, and the whole forming a more or less compact and flat-topped group. Often two or three thinner and more delicate spinelets may be seen on the outside of the group. The paxillae diminish in size in the central area of the disk, and in the immediate centre become extremely small and indistinguishable as separate paxillae ; there is also a more or less strongly marked tendency to develop a conical prominence in the centre of the abactinal area. The paxillae gradually diminish in size as they pro­ceed along the ray, and become very small at the extremity. A more or less clearly defined arrangement in transverse lines is discernible at the Bidet; of the paxillar area along the ray, but along the median line no order can be made out, nor are the paxillae there distinguished by any difference in size.

The supero-marginal plates, sixty-two in number from the median interradial line to the extremity, are confined entirely to the lateral wall of the ray; they are high and short, the height being more than twice the length. On the inner part of the ray the height is even three times the length and sometimes more. The surface of the plates is covered with small, uniform, squamiform papillae, covered with membrane, which are more or less closely oppressed to the surface, their direction being upward, or towards the abactinal surface; and the squamules along the margins of the plate have a slight inclination over the suture-line, which is thus emphasised superficially. At the extreme abactinal edge of the plate is a short, conically-outlined but flat and squamule-like spinelet, very little longer than broad, which is directed perpendicularly to the plane of the abactinal surface, and to which this series of spinelets forms a well-defined boundary.

The infero-marginal plates correspond exactly to the superior series, their length is the same and their height subequal to those above described, or slightly less on the inner part of the ray. The surface of the plates is covered with squamule-like papillae similar to those on the supero-marginal plates, and there is a similar small, short, flat, pointed and squamule-­like spinelet close to the abactinal edge of the plate; the series of these forming along the ray a longitudinal line which indicates the division of the superior and inferior series of plates. At the actinal edge of the plates is a slightly longer, though still very short, flattened and lancet-like spinelet; and the series of these is likewise continuous throughout the ray. In large examples, on the innermost ten or twelve plates, may be found a short, flattened, squamule-like spine similar to those above described, placed on the middle of the plate, both in the superior and inferior series, forming two exactly intermediate short incipient series.

The adambulacral plates are rather large and long, with a prominent angular margin towards the furrow. Their armature consists of an inner or furrow aeries of five remark­able spinelets enveloped in membrane. The median spinelet is the longest, and is directed over the furrow at a right angle to the direction of the ray. It. is compressed laterally, more or less curved, often truncate at the extremity, and resembles a scimitar with broad and flaring blade. The spinelet on each side is usually peculiarly curved, suggest­ing the shape of a boomerang, and is flattened in the direction of the ray and has its convexity turned towards the median spinelet. Often these spinelets appear clavate in consequence of the fulness of the membranous sac in which they are encased. The outer spinelet on each side is rather smaller than those just described, and has with its investing membrane a distinctly papilliform character. On the actinal surface of the plate are normally three longitudinal series of flattened squamiform papillae, covered with mem­branous sacs, all of uniform height, often somewhat irregular in position, and with three to six in each series, the lines being often badly defined. These papilla are similar in character to, and indistinguishable from, those on the actinal intermediate and infero­marginal plates: indeed, when viewed superficially, it is usually impossible to say where the one series ends and the other begins.

The mouth-plates are elongate and very narrow, the united pair having a long nisi-form outline. Their armature consists of a marginal series of sixteen to eighteen short, cylindrical, obtusely-rounded papilla) on each plate, increasing slightly in length as they approach the anterior end; the innermost two are larger than the rest, often more or less curved; and the innermost frequently has a boomerang form with its convexity directed towards the corresponding spinelet of the companion plate, producing a character at once conspicuous and remarkable. On the actinal surface of each plate is a single uniserial row of low, broad, round-topped, transversely compressed, and closely crowded papillae. About sixteen or seventeen papillae are in each aeries, and their breadth is nearly as great as that of the plates; the marginal series above noticed stand vertically and are appressed to the side of the superficial series. All the papillae are invested with membrane.

The actinal interradial areas are narrow, but are occupied by numerous small imbricating intermediate plates; and these extend nearly to the extremity of the ray, in the form of elongate narrow plates intermediate between the adambulacral and infero-marginal plates. They bear skin-covered papillae precisely similar to those on the adambulacral and inferomarginal plates, the whole forming a uniform covering which prevents the possibility of distinguishing the individual plates on superficial examination. Some of these papillae appear to simulate incipient pedicellariae, but their character is not well defined.

The madreporiform body is small and oval, situated a little on the outer side of a point midway between the margin and the centre of the disk; and its surface is grooved with comparatively few, narrow, highly convoluted, and irregular striation furrows, wide apart, causing it to have a rather coarse appearance.

Colour in alcohol, a bleached yellowish grey; sometimes with traces of a dark brown or faded purple shade on parts of the paxillar area, which lead to the inference that that might probably have been the original colour.

Locality.—Station 311. Off the western coast of South America, near the entrance to the Strait of Magellan, opposite Port Churruca. January 11, 1876. Lat. 52° 45' 30" S., long. 73' 46' 0" W. Depth 245 fathoms. Blue mud. Bottom temperature 46°0 Fahr.; surface temperature 50°0 Fahr.

Remarks.—The peculiar skin-covered papillae or spinelets, regarded by Dathelssen and Koren1 as sessile pedicellariae in their admirable description of Bathybiaster pallidus; seldom seem in this species to appear so pedicellaria-like, or to be so highly specialised, as those described in the North-Atlantic form. I have therefore refrained from speaking of them definitely as pedicellariae in the foregoing description. When, however, the voluminous membranous sac which invests each papilla or spinelet is folded, or injured at the extremity by slight breakage, the superficial resemblance is very striking to the structures described by the Norwegian naturalists. In some of the examples before me, the injury to the sac (in the form of a transverse slit) appears to have taken place during the life of the animal, probably by abrasion when passing over some rough surface, and the margins of the break are somewhat thickened, as if during subsequent healing.

The median spinelet in the furrow series of the adambulacral armature is not so highly specialised in the Southern as in the Northern form. It distinctly simulates, however, its character, the spinelet being covered with an extensive sac in which sometimes a supple­mentary calcareous lamina is present. The papulae on the paxillar area may, under favourable circumstances, be well seen in the Southern form.

Notwithstanding these differences, after reviewing its structure as a whole, I feel little hesitation in referring the present species to Danielssen and Koren's well-defined genus Bathybiaster.

1 Loc. cit., pp. 90-92.

(Sladen, 1889: 240-242)