432 PROF. F. J. BELL ON PONTASTER TENUISPINIS. [May 17, 



15 millim. wide at its base, and 7 millim. wide midway between the 

 base and the tip, while an example of " A. tenuispinus," named by 

 the authorities of the Bergen Museum, gives with a base of 15 millim., 

 a width at the middle of the arm of 5-5 millim., or "35 millim. more 

 than would keep the proportions of width of base to width at middle 

 equal in the two specimens. 



A reference to the diagrams A and B (p. 431), in which the 

 proportions above given are plotted out, will show that in A the 

 " attenuation " is more marked than in B, but the difference is so 

 slight that I submit it is not of the least importance. 



The original diagnosis goes on " margine alto " ; these words at 

 first presented some difficulty, as several of the specimens before me 

 are remarkable for their flatness ; but the matter was quite cleared up 

 when I sent two specimens to Prof. Quennerstedt, in order to have 

 them compared with the type in the University of Lund. 



These specimens, which I marked A and B, had respectively 

 R=76 and 51, while the depth of the body at the angles was 3'3 

 and 4*3 millim. each. In A the supero-marginals merely formed a 

 fringe to the upper surface of the arm, while m B they formed 

 a distinct line on the upper surface. 



In Prof. Quennerstedt' s opinion A cannot be A. tenuispinis, while 

 B is certainly very close to it. In other words, A. tenuispinis, as 

 represented by its type, by the figure given by its describers, and by 

 the sketch sent me by the learned Professor of Lund, has the supero- 

 marginals " forming a well-defined border on the abactinal surface." 

 But this is the chief character by which P. limbatus, Sladen, is 

 distinguished from P. tenuispinus, Sladen (D. & K., sp.). As 

 a matter of fact, however, this is a point in which specimens brought 

 from the same locality vary very greatly, and every stage may be 

 found between that in which the supero-margiual is a mere line on 

 the upper surface, and that in which it forms a distinct band. 



If, however, there are to be specific distinctions made, it is clear 

 that P. limbatus of Sladen must be regarded as the equivalent of 

 P. tenuispinis, D. & K. ; and P. tenuispinus, Sladen, must receive 

 a new name. For this, however, I see no real need. 



Recent observers have been struck by the presence at the base of 

 the arm, on the upper surface, of a perforated area, which it has 

 been proposed to call a " papularium," as the tubes which project 

 through the holes are "papular" or respiratory tubes. At times 

 these " papularia " are so distinct that one wonders how it is that 

 they can have escaped the notice of earlier observers ; but the truth 

 is that sometimes the papularia are very indistinct, and at times they 

 cannot be seen at all. This is another very remarkable instance of 

 variation. 



If the views which I hold are justly derived from the facts 

 which I have cited — facts, I may point out, which any worker 

 during the last ten years might have acquired for himself without 

 any overdue call upon his time — the synonymy of this species may 

 briefly be cited thus : — 



