After seeing the photos of Arthroceras fulvicorne from the Spencer Entomological Museum web site...and noticing the yellowish antennae of both the male and female (which, BTW, they mixed-up in regards to labelling)...it occurred to me that the specific epithet "fulvicorne" likely refers to the "tawny antenna" (from (1): fulvi- = L. for reddish-yellow, tawny; and corne = L. for horn).

Further support for that notion is provided by carefully reading the discussion & detailed description of A. fulvicorne on pp. 52-53 and 56 in Webb's 1987 revision of Arthroceras, where the antennae of A. fulvicorne (like much of the body) are described as having ground color dark-yellow to brown, covered with yellow pollinosity & short-appressed yellow setae.

In contrast, note that the antennae in the photo here are more dark brown to blackish, rather than the orange-yellow color seen in the Spencer Museum photos of Arthroceras fulvicorne. (Note also that the apical flagellomere in each of those A. fulvicorne photos is relatively short & stout compared to the longer & more narrow apical flagellomere here...which, as noted below, is a diagnostically critical distinction!)

Moreover, the location here lies outside Webb's distribution map for A. fulvicorne which (in its western portion) hugs the Sierra Nevada-Cascades mountain axis in CA and OR, expanding across the Columbia Plateau in WA and ID, and continuing along the Rockies from BC-AL to CO.

In contrast, the location of this post is in the Coast Ranges of OR, which lie within the area given in Webb's distribution map for A. pollinosum...a species with dark-brown to black (or "fuscous") antennae, as in the photo here.

The most critical characters for separating the species in Webb's key are: 1) the length-to-width ratio of the apical flagellomere; and 2) for females, color (i.e of the head, antennae, thorax, abdomen, etc.). While image resolution in the photo here is insufficient to clearly see the delimitation & proportions of the individual flagellomeres...nevertheless, based on what can be discerned, my impression is the apical flagellomere is more than twice are long as wide (but not so narrowly thread-like as to be "stylate"). So my best shot for species ID would be A. pollinosum...compare with the photos in the post below, where the individual flagellomeres are discernible:

   

The other two nearctic species of Arthroceras recognized in Kerr(2)(2004) (the most recent major publication on Rhagionidae I've been able to access up to now) are A. leptis and A. subaquilum...apparently they have ranges to the north of the location here (though the edge of the range for A. leptis is very close, see Webb's A. leptis distribution map. Webb gave no map for A. subaquilum, since he synonymized it under A. fulvicorne, though he mentioned a single record from Banff, Canada fairly far to the northeast.)

Thank you!