100 PROCEEDINGS OF THE NATIONAL MUSEUM vol.84 



cision. The same may be said of the numbers and positions of the 

 crests on the segments that precede those with the full number. 

 One of the primary crests and one of the secondary crests are 

 omitted on each side of the middle, leaving only 10 dorsal crests, 

 instead of 14, which occasions the abrupt transition. 



Not only the numbers of crests are constant, but the transition to 

 the full number of crests takes place regularly on the same segment 

 in each species. That the point of transition should be constant in 

 the species would hardly be expected in view of the fact that the 

 transition occurs in a rather wide range of positions in the different 

 members of the family. The precision in this character seems very 

 remarkable, since no variations have been found in the material 

 examined. The reduced numbers of crests on the anterior segments 

 may be considered as a specialized feature, in view of the fact that 

 the transition occurs in different segments in several of the species. 

 Eight different points or positions where such transitions occur have 

 been found among the species now recognized in North America, on 

 segments 8, 9, 11, 12, 16, 17, 18, and 19. That no specimens have been 

 found with transitions at segments 10, 13, 14, and 15 is a further 

 indication of definite specialization. Moreover, the species of the 

 same genus, though not changing the number of crests at the same 

 point, are very closely grouped, within a range of only one point in 

 each case, as 8 and 9, 11 and 12, 16 and 17, 18 and 19. The species 

 that carries the reduced number of crests farthest back, that is, includ- 

 ing segment 18, has the body slenderer than the others and also has 

 narrower and less prominent crests. 



Shortening of the secondary crests, in addition to the reduction 

 in size, may be considered as a further stage of specialization. In 

 two of the western genera, Diactis and Tynomma, the secondary 

 crests are like those of Spirostrephon in being as long as the pri- 

 mary crests and attaining the posterior margins of the segments, 

 while in the two other genera, Golactis and Heptium, the secondary 

 crests do not reach the margins of the segments, and may be only 

 half as long as the primary crests. On the anterior segments of 

 CoUctis and Heptium the tendency to reduction of the secondary 

 crests is carried still further. On the first four segments the second- 

 ary crests have become obsolete, or are represented by very short 

 rudiments, while on the first segment they are entirely absent. Thus 

 in Colactis and Heptium the first segment has only 10 crests, con- 

 trasting with 16 or 18 crests in Tynoimna and Diaetis. The reten- 

 tion of the larger numbers of crests on the anterior segments is an- 

 other indication of the affinities of Tynomma and Diax^tis with Spiro- 

 strephon, rather than with the other western genera. 



The shortening of the secondary crests in Colactis and Heptium 

 is occasioned or accompanied by an excavation of the surface of the 



