Western Black-legged Tick, Ixodes pacificus, aka Deer Tick

This tick can be highly variable in appearance; a common morph has a dark shield, reddish abdomen, darker legs and no blotchy spots.
Our other tick, the Dog Tick, usually has no distinct shield on the cephalothorax, a pattern of purplish spots and lighter legs.

Notes on North American ticks from Wikipedia:

Ixodes is a genus of hard-bodied ticks (family Ixodidae). It includes important disease vectors of
animals and humans (tick-borne disease), and some species (notably in Australia) inject toxins that
can cause paralysis. Some ticks in this genus may transmit the bacterium responsible for Lyme disease.

* Ixodes scapularis (formerly Ixodes dammini), known as the black-legged tick or deer tick,
is common to the eastern part of North America and is known for spreading Lyme disease.

* Ixodes pacificus, the Western black-legged tick, lives in the western part of the continent
and is responsible for spreading Lyme disease and the more deadly Rocky Mountain spotted fever.
It tends to prefer livestock as its adult host.

* Dermacentor variabilis, the American dog tick, perhaps the most well-known of the North American
hard ticks. This tick does not carry Lyme disease, but can carry Rocky Mountain spotted fever.