What about Nycteus infumatus? The antennal segments on my beetle seem too long for Euscaphurus spinipes, according to American Beetles. American Beetles does say Euscaphurus spinipes is the only Eucinetid that occurs in CA, but Essig has 8 Nycteus infumatus from my county and 8 others from 2 neighboring counties (still listed as Eucinatus in the museum...).

There are no E. spinipes at Essig, but there are 2 E. saltators from my county. IDs of those 2 are suspect, apparently.

All the Eucinetids at Essig look more or less the same to me, no matter what the genus.

I'm going to try to catch another of these beetles. I had 2 come to the light Saturday night, and if I'd known what I know now, I would have caught the 2nd one too and gotten a ventral shot (under more careful confinement, etc.).

According to the key Euscaphurus is separated from Nycteus/Eucinetus by the fourth meta- and mesotarsomeres being shorter than the fifth - which is clear in your lateral view, and the basis of my placement (I don't have any specimens).

However, I do have Vit's paper, and his description of E. spinipes does not match up as well as I would like. E. saltator does have short, thick antennae, while E. spinipes has longer antennae, but from the desciption not quite that long as your specimen. Looking at Hatch's original illustration of E. fumatus, it does appear that the fourth tarsomere is shorter than the fifth in that species, and sort of trashes that character for separating these two genera. Nycteus infumatus does look correct.

California checklist. I'll be interested to see what you decide about this. I've created the family page.

I'm really pretty sure that it isn't a Euscaphurus, and it does match the figure of Nycteus infumatus in Hatch quite nicely.

We have numerous West Coast specimens of eucinetids, all of which have the 5th tarsomere distinctly longer than the 4th. I don't think any of them are Euscaphurus, but there's no way to confirm or refute this without a genuine Euscaphurus specimen for comparison. It's impossible to see the front coxae in most specimens due to the head covering it up, and everything seems to have five large sternites and one itty-bitty sixth sternite, so that character is pretty worthless as well. So, all of the characters in the key are either demonstrably worthless, or lacking any illustrations that would let you actually make a clear decision.