1114 The American Naturalist. [December, 
though in the Iuliform Craspedosomatide the setæ sometimes rise from 
punctations, e. g. Caseya. The last segment is rudimentary and has 
four conic processes like those of the last segment of Craspedosomatide. 
The appearance of sete on the last segment in these diverse forms 
accords with the known fact of their great constancy in the other sub- 
orders in supporting the view that they are primitive characters and 
hence of great importance in classification and the estimation of affin- 
ities. 
The repugnatorial pores are subdorsal, located in the anterior part 
of the segment and oceur in an uninterrupted series from the fifth 
segment. The occurrence of sets and pores on the same animal indi- 
cates that closer affinity may prove to be possible between the Craspe- 
dosomatide and Callipodide than would be indicated by arranging 
them in separate suborders. 
This combination of characters indicates a wide divergence in devel- 
opmental history from the other recent Diplopod types. This diver- 
gence is also indicated by the fact that the affinities of Stemmatoiulus 
are evidently with the carboniferous forms known as Xylobius. The 
segments of Xylobius, according to Scudder’s diagrams, are divided 
into so-called “frusta ” by longitudinal impressed lines not apparently 
comparable to the striz of Iulide nor to the carine of Callipodide or 
Cambalide. Hence I have arranged Stemmatoiulus and Xylobius 
as representatives of suborders® under a new ordinal name, Monocheta, 
coérdinate with the Oniscomorpha, Limacomorpha, Colobognatha and 
other groups noticed below. The comparative study of the Diplopoda 
necessary in examining the question of the proper systematic value of 
the characters presented by Stemmatoiulus has led me to look upon 
the Helminthomorpha of Pocock as a composite group, the different 
members of which are not necessarily more related to each other than 
to the Oniscomorpha or Limacomorpha. 
An apparently satisfactory means of division into groups the mem- 
bers of which have more evident affinity among themselves, is to be 
found in the location of seminal opening and the structure of the 
external seminal ducts when present. Without known exception the 
characters drawn from these organs are accompanied by a definite 
complex of other features so that there appears to be ample ground for 
5 Suborder Xyloiuloidea, to contain the family Xyloiulide, genus Xyloiulus 
fossils from the Sigillarian stumps of Nova Scotia. Xylobius Dawson is pre- 
occupied, and is replaced by Pyloiulus. 
