Hirudisomatid Millipeds 
135 
Mexiconium absidatum, new species 
Figs. 4, 31-38 
Type specimens — Male holotype and two male paratypes (VMNH) 
collected by R. E. Leech, 25 August 1967, 13.3 mi (21.8 km) S La 
Vigas [ca. 18 mi (28.8 km) WSW Jalapa], on the north side of Cofre 
de Perote, Tembladera, Vera Cruz, Mexico. 
Diagnosis — With the characters of the genus (Figs. 31-38). 
Variation — The holotype has 27 segments and is 6.7 mm long; 
the paratypes have 29 and 35 segments and measure 7.7 and 10.8 
mm, respectively. 
Ecology — According to the vial label, the types were collected 
between 11,500 and 13,500 ft., an extremely high elevation for a 
North American milliped. The habitat is not indicated. 
Distribution — Known only from the type locality, in the Sierra 
Madre Oriental in the interior of Vera Cruz (Fig. 1), some 440 mi 
(704 km) south of the Rio Grande and the United States border and 
around 1,200 and 2,117 mi (1,920 and 3,387 km) from the most proximate 
sites of O. gracilipes and O. bivirgata, respectively. 
Remarks — This species, the first record of the family from Mexico, 
is isolated from an ancient dispersal of the Hirudisomatidae across 
North America that extended southward for an unknown distance into 
Mexico. The dorsal branch of its ultimate anterior gonopod podomere 
is similar to those of O. gracilipes/ sierra, instead of the Pacific Coastal 
species, and this is further evidence that the Hirudisomatidae dispersed 
from east to west across North America, as is its occurrence on the 
eastern side of Mexico rather than the west (see below). The specific 
name refers to the vaulted body form. 
Discussion 
The two great allopatries in the Hirudisomatidae merit elaboration 
because they exemplify broader patterns among Nearctic Diplopoda. 
The family’s occurrence in Mexico is evidence of southward expansion 
of ancestral stock, a dispersal also demonstrated by the Glomeridae, 
Spirobolidae, Parajulidae, Nearctodesmidae, and Xystodesmidae. The 
Mexican xystodesmid fauna exhibits Appalachian affinities and also 
immigrated from the east-Nearctic (Hoffman 1969), but the austral 
representatives of the other families have west-Nearctic relationships 
and occur more in western Mexico, aside from cave inhabiting species 
of Glomeroides Chamberlin (Glomeridae) in Nuevo Leon, Tamaulipas, 
and Vera Cruz. Like the Hirudisomatidae, the species of the Spirobolidae, 
Parajulidae, and Nearctodesmidae show further evidence of Nearctic 
ancestry in their occurrences at high elevations, in “Nearctic” environments, 
