MILLIPEDS OF FAMILY LYSIOPETALIDAE LOOMIS 101 



segments along the posterior margin, between the primary crests. In 

 some of the species the surface is lowered very abruptly at the ends 

 of the shortened secondary crests, forming a row of small depressed 

 scallops or recesses along the posterior margin of the segment. In 

 one of the species of Heptium the stepping-down of the surface of 

 the segment is very abrupt, and the depressions appear to be bordered 

 in front by a narrow rim, as if the secondary carinae had divided to 

 form the excavated areas. A similarity may be noted between such 

 bordered areas and the ring of channeled or fluted sculpturing of the 

 surface around the posterior half of the anterior subsegments, which 

 suggests that the theory of metaphanic variation may apply. 



LATERAL CRESTS 



Lateral crests also are found in all the genera of Lysiopetalidae, 

 below the crests or carinae that bear the repugnatorial pores. In all 

 the American genera two of the lateral crests are much more prom- 

 inent than the others and larger than the dorsal crests. In the genus 

 Spirostrephon two of the lateral crests are especially large and as 

 prominent as the poriferous crest, so that a lateral view of these ani- 

 mals gives the impression of three conspicuous lateral crests of nearly 

 equal size. In some of the western forms the lateral crests are less 

 developed, but detailed comparisons have not been made. 



POSITIONS OF DORSAL BRISTLES 



The regular sculpturing of the segments is supplemented by a 

 series of bristles located in definite patterns and in specialized rela- 

 tions with the crests. All the primary crests, both dorsal and lateral, 

 as well as the pore-bearing carinae, are subtended by bristles, one 

 bristle associated with each crest. Except on a few of the anterior 

 segments, all the bristles are inserted along the posterior margins of 

 the segments, not directly at the ends of the crests but slightly out of 

 line with the crest, on the side toward the middle of the body. No 

 deviation from this posterior position of the bristles is found in any 

 of the pore-bearing segments, including segment 6, where the pores 

 begin. 



A different arrangement of the bristles appears on segment 5 (fig. 

 18, k)^ where only six bristles are found along the posterior margin 

 of the segment, three on each side of the median line, at the second, 

 third, and fifth crests. As if to replace the missing bristles of the 

 first and fourth crests, two other bristles are found at the front of 

 these crests, and on the opposite side of the crests, away from the 

 middle of the segment. On segment 4, all the bristles have the ante- 

 rior position, like the two bristles that stand in front on each side of 

 segment 5, and this arrangement obtains also on segments 3 and 2. 



