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Genus Cybaeus

brown spider found in leaf litter - Cybaeus - female Difficult spider - Cybaeus reticulatus - female Another Cybaeus sp. - Cybaeus signifer - male Another Cybaeus sp. - Cybaeus signifer - male Cybaeus sp. - Cybaeus eutypus - female Cybaeus - Cybaeus reticulatus - male Cybaeus male - Cybaeus signifer - male Cybaeus male - Cybaeus signifer - male
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Chelicerata (Chelicerates)
Class Arachnida (Arachnids)
Order Araneae (Spiders)
Infraorder Araneomorphae (True Spiders)
No Taxon (Entelegynae)
Family Cybaeidae
Genus Cybaeus
~61 species in BugGuide's range (North America north of Mexico)(1)
(Information from Dr. Robb Bennett, from a comment here.)
Females of the 3 eastern NA species are very difficult to tell apart. When possible, you can rule out C. patritus on basis of distribution; but separating C. silicis from C. giganteus is primarily a value judgement based primarily on size and somewhat on leg banding: carapace length in C. giganteus females ranges from 4.5 to 5.4 mm (a relatively large spider), in C. silicis females the carapace length range is 3.4 to 4.0 mm (not really a lot different from C. giganteus!).

Additionally, C. silicis legs are usually at least faintly banded while those of C. giganteus usually show no evidence of banding.

The BEST way to get positive ID of your Cybaeus is to try to find mature males -- these are usually only found later in the summer and through the fall until the weather turns (you can usually find females year-round). Males are much easier to identify than are females and males of the 3 eastern NA species are quite easy to tell apart, even from digital images as long as you have a clear shot of the male pedipalp. You can usually ID a mature male using a good hand lens although using a dissecting microscope is the best way.
(Information from Dr. Robb Bennett, a combination of pers. comm. in July 2011 and also from a comment here.)
This might be re-ordered to include a full list of all known species when time permits but, as of right now, these are the ranges that are easily accessible:

Almost all Cybaeus species occur in woodlands of western North America (the majority in CA and southern OR).
    Species specifically recorded from western Washington:
        C. reticulatus
        C. morosus
        C. scopulatus
        C. bulbosus
        C. signifer
        C. eutypus
        C. perditus
    Species recorded from Canada & Alaska(2):
        C. bulbosus: BC
        C. eutypus: BC
        C. morosus: AK, BC
        C. reticulatus: AK, BC
        C. shoshoneus: BC
        C. signifer: AK, BC
        C. sinuosus: BC, AB

Only 3 species of Cybaeus occur in woodlands of eastern North America (C. giganteus & C. silicis are relatively common within their range):
    C. giganteus: ranges from NY to GA & AL.
    C. patritus: restricted to the southern Appalachians in TN and NC.
    C. silicis: ranges from OH and MD to NC.
Internet References
~ - Cybaeus tius (=C. reticulatus) male from WA.
~ - Cybaeus eutypus male from WA.
Works Cited
1.Spiders of North America: An Identification Manual
D. Ubick, P. Paquin, P.E. Cushing and V. Roth (eds). 2005. American Arachnological Society.
2.Checklist of the spiders (Araneae) of Canada and Alaska
Paquin, Buckle, Duperre, & Dondale. 2010. Zootaxa 2461: 1–170.