This species used to be widely known as A. ostoyae. However, this name was deprecated in favor of A. solidipes based on the following paper:

Burdsall, H. H., Jr., and T. J. Volk. 2008. Armillaria solidipes, an older name for the fungus called Armillaria ostoyae. North American Fungi 3(7): 261-267. doi: 10.2509/naf2008.003.00717 Published August 29, 2008

You can read all the details in this paper, which is available here as a pdf file.

This link was provided and is discussed more on Tom Volk’s Mushroom of the Month for September 2008.

Here’s the original description from Peck as given in the above paper:

Armillaria solidipes:
Pileus fleshy, convex, even, glabrous, tawny or yellowish brown, tinged with red in drying, flesh whitish; lamellae rather close, adnate or slightly decurrent, white or whitish; stem long, firm solid, colored like the pileus. Pileus 2.5 – 5 cm. broad; stem 10 – 25cm. long, 6 -12 mm thick. Densely cespitose. About spruce stumps. Colorado. September. E. Bartholomew. Spores not seen.

There was also discussion of this taxon on this site under the name A. ostoyae. Here’s a revised version of that discussion:

Paraphrased from Darvin’s comment on Observation 4530:

“In California, both Armillaria mellea and A. solidipes have a thick, felty annulus and they are the only Armillaria species here to have that character. They differ by the color of both the cap and the annulus. A. mellea has a white to yellow edge on the annulus, the cap is honey yellow and the disc is darker than the margin giving it a two-tone appearance. The cap is smooth and the hairs are indistinct. A. solidipes has a brown edge on the annulus, the cap is brown with dark hairs.”

Another quick rule is that A. mellea tends to grow on hardwoods and A. solidipes on conifers. Tom Volk definitely id’ed a collection from southern California growing on hardwood as a slightly unusual form of A. mellea. I also sent him a sample from Santa Cruz that came out as A. mellea. Apparently the California material mated with the true A. mellea, but it was not a typical mating. Macroscopically I remember Tom noting that the California material has a scalier cap than classic A. mellea (which is smooth). Based on his key it looks like microscopy and and in particular looking for clamps at the base of the basidia is the only way to be sure. Personally I have a hard enough time find a good basidia, much less figuring out whether there’s a clamp at the base.