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Family Paradoxosomatidae

Flat-backed Millipedes mating - Oxidus gracilis - male - female Mating millipedes under bark - Oxidus gracilis - male - female centipedes - Oxidus gracilis Millipedes - Oxidus gracilis Oxidus gracilis Unknown Milliped - Oxidus gracilis millipede?? - Oxidus gracilis Oxidus gracilis
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Myriapoda (Myriapods)
Class Diplopoda (Millipedes)
Order Polydesmida (Flat-backed Millipedes)
Family Paradoxosomatidae
>700 spp. worldwide
Five introduced species in North America.
"For practical purposes, the family Paradoxosomatidae can be recognized by the dorsal transverse grooves on the 'metazona' of each segment." -- Rowland Shelley

Not native to North America; introduced species Oxidus gracilis is widespread.
Known species in US and Canada: (1)(2)

Akamptogonus novarae - California
Asiomorpha coarctata - Gulf Coast states.
Oxidus gracilis - Widespread, probably all states and provinces

See here for comparison of A. coarctata and O. gracilis.
Internet References
Rowland Shelley: Photos of Paradoxosomatidae. The Myriapoda (Millipedes, Centipedes) Featuring the North American Fauna.
Works Cited
1.Checklist of the millipeds of North and Middle America
Richard L. Hoffman. 1999. Virginia Museum of Natural History Special Publications.
2.Annotated checklist of the millipeds of Florida (Arthropoda: Diplopoda)
Rowland M. Shelley. 2000. Insecta Mundi, 14(4), 241–251.