786 



PROFESSOR J. STEPHENSON ON A RULE OF 



preceding species. Arranging the figures as before, and giving the actual lengths 

 of the distal segments in m : — (i) 45, 42, 32 ; (ii) 42, 35, 28. 



The difference between the two ends of the bundle is thus extremely marked. 

 Further forwards, e.g. in segment xx, it is less marked : — 43, 39, 36, 36. Further 

 forwards still, the proportions at the two ends of the bundle become equalised and 

 finally reversed; thus in segment xiii, where there were only two setae in the bundle, the 

 distal segments of the inner and outer setae respectively were 52m and 50 m in length, 

 and the proportions to the length of the whole seta '49 and '48. In ix reversal had 

 taken place : — 49 m, 54 m, 56 m (three setae in bundle), or "48, '50, "51 of total length. 

 In viii the relations were similar (two in bundle) : — lengths of distal segments (inner 

 and outer respectively) 49 m and 52 m, proportion of length of seta '48 and '50. Thus 

 in the anterior part of the body the inner setae of the bundle have the distal segment 

 shorter than the outer setae, both absolutely and relatively to the total length. 



In another group of species represented by Nais communis var. punjabensis and 

 Naidium minutum the innermost seta of a bundle has as a rule a longer distal segment, 

 relatively and absolutely, than the outermost ; but there is no very regular series 

 of intermediate steps from one to the other. They thus differ in this latter respect 

 from the foregoing species ; the condition is to some extent similar, but less marked. 

 The arrangement of the figures is as above, from inner to outer. 



39 p\ 

 '43. (" 



Nais communis, Piguet, var. punjabensis. 



Four instances may be given : — 



(i) Portion distal to nodulus, in p, 43, 41, 43, 43. 

 Proportion of length of seta, '49, '45, '44, '44. 



(ii) 45, 41, 41, 41 fi) (iii) 48, 37, 41, 



= '53, '45, '45, '45. \ ='58, '44, '46, 



(iv) 41, 37, 39 /*) 

 = •49, '45, '45. ) 



Naidium minutum, Stephenson. 



Two examples may be given : — 



(i) 22, 21, 22, 21, 19 fi) (ii) 25, 25, 23, 21 , 



= •44, "42, '43, '42, '38. \ ='43, '44, '41, "40. 



Finally, in a form which I have identified with Pristina sequiseta, Bourne, the 

 peculiar relation is scarcely recognisable : — 



(i) 22, 21, 21, 19, 20, 20 p\ (ii) 22, 25, 25, 



= •44, '42, '44, '38, '42, '42. \ ='41, '44, '44, 



(iii) 23, 23, 23 p\ (iv) 25, 23, 21, 21, 21 /n) 



= "46, '46, '46. ) ='47, '43, '41, "41, '42. { 



5, 22, 21 p) 

 L, -41, '42. J 



