I’m about to leave the office for a few days, and this may be the last note I can post on this subject for a bit. In response to your question, I went back to Dr. Bas’ 1969 discussions of the two species. He notes their similarity as you have done. There are even more similarities; for example, they both have clamps at the bases of their basidia.
Bas’ grouping of species into stirpes (singular, “stirps”) and of stirpes into subsections is based on characters such as clamps, microscopic structure of the volva, size and shape of spores, nature of the pileipellis, macroscopic form, etc. He gave a great deal of attention to the structure of the volva in his groupings of species.
In the present cases, he notes several differences and, in the end, falls back on microscopic examination of the volval structure: “In cases of doubt the microscopical structure of the base of warts has to be studied; in A. ravenelii the tissue there consists nearly completely of hyphae and only scattered, sometimes very large, inflated cells.” He previously mentions that rhopalopus has no clearly segregated base in its warts which comprise hyphae and inflated cells that are soon disordered and, hence, may form warts but not cleanly pyramidal ones.
I am going to post all of Bas’ microscopic information for ravenelii on the web site. I will also post his comments on both species. I don’t know if I am going to get to this action this morning before I have to leave. I did notice these points, which have not come up in our discussion (and which I’ve never used myself…to date, but will check into): (1) A. rhopalopus is described as having a rather thick pileipellis that sooner or later gelatinizes at the surface. (2) The fibrillose base of the warts in A. ravenelii is said to be intimately connected (by continuous hyphae) to the flesh of the cap (apparently passing through the pileipellis, which is mentioned as present but without a described thickness). Yes, the pileipellis descriptions are not point-to-point comparable as Bas recorded them.
In summary, at present we are reliant upon Bas’ 1969 monograph (which is ground-breaking and brilliant in my opinion) and, hence, we are dependent on microscopic analysis of the bases of the warts. And this holds even if (as is possible) that Bas (working only with dried material in the relevant case) mixed rhopalopus with the provisionally named taxon “A. magniradix,” which may have been true.
Your last photograph (of warts near the cap margin) is consistent with Bas’ remark regarding ravenelii that the volva consists more prominently of the lower fibrillose layer when you observe it closer to the cap’s margin.
In your material, it appears to me that the drying of the volva collapsed the tissues in the warts near the center of the cap, but the lower level of the volva seems to be distinct, especially near the margin. Without a microscopic “check up,” I’m inclined (like you) to consider ravenelii as more probable than rhopalopus and am now going to adjust my votes on the various names.