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Numerous on underside of rock in perennial creek. Found in pockets/pits of volcanic rock.

Water Mites of North America - Superfamily Lebertioidea Photo Library

Not firm on this; it wasn't keyed out. Similar to this one from boldsystems.org.

On coyote bush (Baccharis pilularis).

The lateral carinae are long and terminate very close to the hood. Height of the hood is tall. See discussion here: https://www.inaturalist.org/observations/4110532.

https://bugguide.net/node/view/1100357

Found inside of several Cryptoporus volvatus conks (https://www.inaturalist.org/observations/56287389) on a fallen pine in coniferous forest dominated by Douglas fir.

2 mm in length

Plesiocis cribrum Casey

Geographic range.-Northern and montane regions of North America from British Columbia to the Gaspe Peninsula of Quebec, south to the Laguna Mountains in California, the edge of the Colorado Plateau in Arizona and New Mexico, the Great Lakes Region, and the Shenandoah Mountains of Virginia (map 12).
California records.-Throughout most of the Boreal Zone areas of the state, from coastal and foothill Transition Zone situations to above 7,000 feet in the central Sierra Nevada and above 8,000 feet in southern California.

Host fungi.-Polyporus volvatus [Cryptoporus volvatus]; Polyporus anceps'; Polyporus alboluteus.

Discussion.-This species is characterized by the robust form, 9-segmented antennae, dentate protibial apex, coarse, dense, and confused punctation, and vestiture of short bristles. The male has 4 sharp teeth on the frontoclypeal ridge and 2 weak protuberances on the apex of the pronoturn. Cis vitulus may be confused with P. mibrum on the basis of size, general form, and the sexual modifications, but individuals of the former are distinguished by the 10-segmented antennae, finer punctation, especially on the pronotum, broader lateral pronotal margins, and lack of an abdominal fovea in the male. In Dolichocis manitoba the antennae are 9-segmented and the punctation is similar, but that species differs in the more elongate form, rounded protibial apices, and modifications of the head in the male.
Plesiocis cribrum is very common throughout western North America wherever conifers occur and is also known from the northeastern states. Although the preferred host is Polyporus volvatus, the species also breeds in Polyporus anceps-the two fungi often grow together on dead conifer logs.

(The above is from THE CIID BEETLES OF CALIFORNIA - BULLETIN OF THE CALIFORNIA INSECT SURVEY - VOLUME 17 - JOHN F. LAWRENCE [1974] [https://essig.berkeley.edu/documents/cis/cis17.pdf])

Photos - https://www.zoology.ubc.ca/entomology/main/Coleoptera/Ciidae/Plesiocis.php