THE MYRIAPODA OF NORTH AMERICA. 195 



long and filiform ; but in the genus Spirobolus they are generally short and massive. The 

 body is cylindrical and composed of very numerous segments, which are more or less imbri- 

 cated. The line of conjunction of the two subsegments is almost, if not quite, always well 

 marked. The organization of this family is more fitted for activity and their habits less 

 sluggish than in the Polydesmida?, although less so than in the Spirostrephonidse. The 

 coloration varies, some species being ornamented with annuli, others with stripes. The 

 only surface markings, in our American species at least, consist of longitudinal channels, 

 or flutings and punctations. 



The American species are comprised in two genera, lulus, .Spirobolus. Some authors 

 recognize a third genus, Uncigera, founded on the presence of a mucro or point on the 

 posterior scutum. Now a suite of specimens shows this at once to be such a gradually 

 developed character that it is almost impossible to say where the last scutum is simply 

 acuminate, and where it is slightly mucronate. Besides, species which appear in every 

 other respect to be very closely allied would be thrown into different genera, were this 

 Uncigera to be recognized as such. 



The specfic characters are drawn: 1. From the number of segments composing the 

 body. 2. From the coloration and surface markings. 3. From the last scutum. 4. From 

 the genital appendages, besides other minor points, such as differences in the antenna?, 

 shape of the head, &c. 



The number of segments is apparently confined within a small limit in the adult of each 

 species; but in applying this character, it must be remembered, that the young have less 

 than the normal number. Where there is anything peculiar in the pattern of coloration, 

 it appears to be persistent in the species. Surface markings more generally characterize 

 small groups than individual species, yet are sometimes available even in closely allied 

 forms. The possession of a mucro characterizes groups ; whilst its size and its shape are 

 often specific. As in all the other Diplopoda, each species has a peculiar and persistent 

 form of the Genitalia, more marked in the male than in the female. Indeed a certain 

 type of arrangement seems in this group to characterize a genus. 



Genus lULUS. 



Scutum priniuin magnum, lateribus in femina nonnihil productis. Scuti secundi latera baud producta. 

 Sides of the first scutum somewhat produced in the female. Sides of the second scutum not produced. 

 None of the North American species of this genus approach at all in size to the Spiro- 



in i 



li. The body is always slender, and seldom more than three inches in length. The 

 head is distinct, and has the antennae always more or less elongate and filiform, apparently 



