140 
Rowland M. Shelley 
than remnants that remained in the original source area. Octoglena 
conforms to this diversity pattern, but the eastern component is sister 
to the western species, implying dispersal from east to west. The overall 
distribution of the family, with the greatest diversity in Europe, supports 
this scenario and suggests a Laurasian origin for the New World fauna. 
Futhermore, the continuous, parapatric ranges and the absence of extinc- 
tions along the Pacific Coast contrast so strongly with the allopatries 
and lacunae elsewhere in the New World that they probably reflect 
recent evolution and the end products of range expansion. Octoglena 
therefore represents the northern part of an Ancient Holarctic faunal 
assemblage that probably spread from east to west across the United 
States, and southward into Mexico, in one or possibly two dispersals. 
There has been considerable extinction, as evidenced by the extensive 
lacunae; that between O. sierra and O. gracilipes is greater than those 
in other disjunct taxa in the United States (Table 7), and coupled 
with the unique somatic features of O. gracilipes , implies lengthy 
isolation of the eastern and western hirudisomatid faunas. 
Hoffman (1969) cited three distribution patterns that have impacted 
Appalachian Diplopoda — Ancient Holarctic, Tertiary Nearctic Endemi- 
city, and Late Coenozoic Austral Immigration. These also affected 
lowland areas, and “West-Nearctic Immigration”, from beyond the 
Continental Divide, seems equally important in understanding the origins 
of the fauna east of the Plains. Causey (1974) postulated a west- 
Nearctic origin for the Parajulidae, with dispersal centers in central 
California and around Puget Sound that gave rise to the east-Nearctic 
and Mexican/Guatemalan faunas. This hypothesis is consistent with 
the conclusion of Shelley (1994c) that the exclusively western julidan 
superfamily Paeromopodoidea probably arose from a parajuloid-like 
ancestor along what is now the border between Oregon and California. 
This is a key area in the evolution of west-Nearctic diplopods (Shelley 
1994c), being the apparent primary centers of evolution of the 
Paeromopodoidea and Scytonotus, and harboring a peripheral relict 
species of Chonaphe Cook (Shelley 1993, 19946, c). The area also 
seems to be a secondary center of evolution within Octoglena , thereby 
accounting for the greater diversity, parapatry, and abundance of the 
Pacific Coastal components. 
ACKNOWLEDGEMENTS — I thank Jonathan Coddington (NMNH) 
for loan of the holotype and paratypes, and G. B. Edwards (FSCA) 
for additional paratypes, of Polyzonium gracilipes; they also loaned 
non-typical specimens from these holdings. The following curators 
loaned material from the indicated collections: N. I. Platnick (AMNH), 
