According to the Greene/ Hearn biography, W. W. Denslow created a cover for the book Gemma, published by Rand McNally in 1898. I have a copy of this title but I'm not completely certain whether it has the Denslow cover. I think it does, but it is a little difficult to say - there is no seahorse signature, which is the easiest way to be certain, but as I've shown several times, the signature is not always present.
The overall look of the cover is a bit different than I would expect. The type used for the author's name certainly looks like Denslow, as does the vignette on the rear cover. I believe many of these titles went through several cover designs in a short period of time, and Denslow wasn't the only artist designing for the firm.
Overall, I believe it is a Denslow cover, but it's an example of how collecting can be confusing - sometimes you just have to take a chance with your own instinct!