102 PROCEEDIK'GS OF THE ]S'ATION.\L MUSEUM vol.84 



The first four segments have no bristles along the posterior margin, 

 but only at the front of the crests. On the first segment the bristles 

 no longer appear in a regular transverse line, but the 10 bristles are 

 still found as a constant number. 



The two positions of the bristles suggest that the ancestral forms 

 of the Lysiopctalidae may have had two or more rows of bristles 

 across the segments, as do some of the other groups of millipeds. 

 The alternative view would be that the bristles belong to a single 

 primitive row, which has changed to these ditferent positions on the 

 segments. The bristles possibly are older than the crests in the 

 phylogeny of the group, so that on the anterior segments crests may 

 have developed behind the bristles but in front of the bristles on 

 posterior segmeiits. Tlie crests may have arisen from the tubercles 

 that bore the bristles in the primitive forms. Although the bristles 

 are now separate from the crests, they seem to be associated definitely 

 with them. 



Under the assumption of two prunitive rows of bristles it would 

 need to be inferred that the posterior row has been suppressed on 

 the anterior segments, while the anterior bristles have been suppressed 

 on the posterior segments. The different positions of the bristles on 

 segment 5 would seem at first to support the assumption of two origi- 

 nal rows, but less confidence is felt in this explanation when account 

 is taken of the strict limitation in the nmnber of bristles and the 

 remarkable precision of arrangement shown at the point of transition 

 from anterior to posterior bristles, on segment 5. If the bristles of 

 this segment are supposed to represent two primitive rows, it is diffi- 

 cult to understand why neither of the rows should ever be complete, 

 or why some of the crests, at least, should not have two bristles. So 

 great an accuracy in the suppression of particular bristles of two 

 original series is as difficult to credit as that the contrasted positional 

 relationships on the different segments have developed in one row 

 of bristles. 



With either view of the development of the bristles a scarcely 

 imaginable accuracy of adjustment of the hereditaiy controls of the 

 characters has to be admitted to account for the constant number and 

 the exact positional relations of the bristles on segment 5, to say 

 nothing of the other segments. That the transition point should be 

 so definitely fixed upon a single segment is sufficiently wonderful, but 

 that it should be held at so definite a point that four of the crests 

 of segment 5 have their bristles in front, while the other six crests 

 have their bristles behind, is a wonder of accurate adjustment of 

 heredity that leaves all our adjectives inadequate. 



