Semi-desert, foothills. Openings. Spring, summer.
Canyons of the Ancients National Monument, April 11, 2010.

This Opuntia would qualify as O. erinacea if we were to key it with Weber's Colorado Flora: Western Slope, for it has spines that are flattened at least at their base.  But, as explained elsewhere, the Synthesis and the Flora of North America indicate that O. erinacea is a variety of O. polyacantha and it does not grow anywhere near the Four Corners area.

Most O. polyacantha pads, such as the ones pictured at left, are circular.  Some of the spines pictured at left are over four inches long (probably indicating that this is O. polyacantha variety nicholii); spines on most O. polyacantha in the Four Corners area are just one or two inches long (O. polyacantha variety polyacantha).

Semi-desert, foothills. Openings. Spring, summer.
Canyons of the Ancients National Monument, April 28, 2006.

Semi-desert, foothills. Openings. Spring, summer.
Canyons of the Ancients National Monument, April 25, 2009.

Everything has its beginnings; these babies are just an inch high.

Semi-desert, foothills. Openings. Spring, summer.
Rim Trail, Dolores, May 22, 2017.

The spiny pad in the lower right of the photograph produced a flower in 2016 and that flower produced the buff colored fruit in the center of the photograph. In a most unusual fashion, new Opuntia polyacantha pads (in the upper left corner of the photograph) are growing out of last year's fruit --  while the fruit is still on the plant.