1892.] PROF. F. J. BELL ON PONTASTER TENUISPINIS. 433 



PONTASTER TENUISPINIS. (Plate XXVI.) 



Astropecten tenuispinus, Duben & Koren, Wet. Akad. Hdlgr. 

 1844 (1846), p. 251, pi. viii. figs. 20-22. 



Archaster tenuispinus, Sars, Norges Ecliin. (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 

 spineiets. A similar, or sometimes rather stouter, spine is borne by 

 each inferomarginal, and one or more of the surrounding spineiets 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 ambu- 

 lacral 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. Pedicellarise present or absent. Delicate spines may 

 sometimes be seen rising from the dorsal paxilliform plates. 



EXPLANATION OP 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. 



