Species:

Sceloporus clarkii

Clark's Spiny Lizard

Some other names for this species:

Clark Spiny Lizard

Subspecies I've seen:
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Sonoran Spiny Lizard
S. c. clarkii
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Plateau Spiny Lizard
S. c. vallaris
Subspecies:

Sceloporus clarkii clarkii

Sonoran Spiny Lizard

On grounds of Arizona Sonoran Desert Museum, Tucson, Pima County, Arizona
Sonoran Spiny Lizard (Sceloporus clarkii clarkii)
Most of the Arizona Sonoran Desert Museum grounds are outdoors, and a variety of native and non-native species live wild there. This young spiny lizard was clinging to a boulder along the main trail through the outdoor part of the museum.
Pima Canyon, Tucson, Pima County, Arizona
Sonoran Spiny Lizard (Sceloporus clarkii clarkii)
This very large and very dark individual tried to hide behind the tree trunk it was perched upon by constantly shuffling to stay on the opposite side of me. Eventually it decided on the safety of some nearby boulders, and I got just a couple of poor photos as it scurried away.
San Pedro Riparian Conservation Area, Cochise County, Arizona
Sonoran Spiny Lizard (Sceloporus clarkii clarkii)
These robust spiny lizards were fairly common in the trees lining the San Pedro river, though they're wary and difficult to approach. Youngsters and females are dark and camouflaged well on tree trunks. Some of the large males are very light with extremely pronounced black collars. We saw two pairs of large adults on different trees, and several individuals scattered about. Unfortunately all the large light-colored males raced up the tree trunks before I could get a decent picture. The sneaky-looking fellow in this picture gives you a hint of the dramatically different appearance of these large males.
City of Rocks State Park, Luna County, New Mexico
Sonoran Spiny Lizard (Sceloporus clarkii clarkii)
Sonoran Spiny Lizard (Sceloporus clarkii clarkii)
This big male spiny lizard occupied a rock a few feet away from a big female spiny lizard. She was enough shyer that I didn't end up with any photos, but he was a lot prettier anyway.
Santa Cruz County, Arizona
Sonoran Spiny Lizard (Sceloporus clarkii clarkii)
Sonoran Spiny Lizard (Sceloporus clarkii clarkii)
While whiling away the oh-so-hot midday hours in anticipation of some exciting nocturnal herping, I chased around a bunch of S. clarkii before I could get one or two of them to stay in place for a few moments.
Subspecies:

Sceloporus clarkii vallaris

Plateau Spiny Lizard

Sedona, Coconino County, Arizona
Plateau Spiny Lizard (Sceloporus clarkii vallaris)
Plateau Spiny Lizard (Sceloporus clarkii vallaris)
I originally mistook these lizards for Desert Spiny Lizards, because I didn't realize that Clark's Spiny Lizards lived this far north. But it turns out that both species live in the red rock desert around Sedona, and the ones I photographed were definitely Clark's. One way to distinguish them is by the dark bands on the forearms, visible in the second photograph here.