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 imbricated. 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 Polydesmidæ, although less so than in the Spirostrephonidæ. 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 or the presence of a mucro or point on the posterior scutum. Now a suite of specimens hows 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 marrings. 3. From the last scutum. 4. From the genital appendages, besides other minor points, such as differences in the antennæ, shape of the head, &c.
The number of segments is apparently confirmed within a small limit in the adult of each species; but in applying this character, it mus 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 IULUS.
Scutum primum magnum, lateribus in femina nonnihil productis. Scuti secundi latera haud 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 Spiroboli. The body is always slender, and seldom more than three inches in length. The head is distinct, and has the antennæ always more or less elongate and filiform, apparently
