| OCR Text |
Show 1892.] PROF. F. J. BELL ON PONTASTER TENUISPTNIS. 433 PONTASTER TENUISPINIS. (Plate XXVI.) Astropecten tenuispinus, Duben & Koren, Wet. Akad. Ildlgr. 1844 (1846), p. 251, pi. viii. figs. 20-22. Archaster tenuispinus, Sars, Norges Echin. (1861), p. 38, pi. 3. figs. 5-7. Pontaster tenuispinus, Sladen, 'Challenger' Rep. Ast. (1889), p. 28. Pontaster tenuispinis, Bell, Ann. & Mag. N . H. iv. (1889), p. 433. Pontaster tenuispinus, var. platynota, Sladen, op. cit. p. 29. Pontaster hebitus, id. op. cit. p. 33. Pontaster limbatus, id. op. cit. p. 35. Distribution. Both sides of North Atlantic, Arctic Ocean, Kara Sea. Largest within the Arctic circle; the Irish forms often very stout. 85-600 fathoms. The following diagnosis of this very variable form may be offered:- The proportion of R (greater diameter) to r (diameter of disc) varies between 3| and 7f. A very variable species in many characters. Disc and arms flat, but the depth of the side at the angles of the disc varying somewhat; the arms taper regularly, and end in rather fine points as a rule. The bases of the arms on the dorsal surface sometimes, but not always, marked by a perforated area (the papu-larium) of an elongate lens-like form containing from ten to twenty holes. The sides of the arms above bounded by a pretty stout supero-marginal, which may, however, be so thin as to be merely a line on the upper surface. There may be as many as 40 supero-marginals, and they pretty constantly each carry a well-developed spine, the base of which is surrounded by a number of spinelets. A similar, or sometimes rather stouter, spine is borne by each inferomarginal, and one or more of the surrounding spinelets may be prominent on account of their length. The intermediate plates on the lower surface are, as a rule, thickly covered with spines, but in these there are, at times, reductions. The spines on the ambulacral plates vary considerably in number and disposition, but the most usual arrangement appears to be a row of about six small spines along the groove with one, two, or three larger spines set transversely. Pedicellariae present or absent. Delicate spines may sometimes be seen rising from the dorsal paxilliform plates. EXPLANATION OF PLATE XXVI. Figures illustrative of the variations exhibited in the size of (I.) the supero-marginal plates, (II.) the characters of the papularia, and (III.) the disposition of spines on the actinal surface and along the ambulacral groove. |
