Should the ending be -anthus?

This is the most defensible wrong spelling on the page, because -anthus really does exist and is common. Dianthus, Helianthus, Agapanthus and Ceanothus all end that way, all from the same Greek word anthos meaning flower, and all pronounced with a plain -us. Somebody who knows those names and then hears cheilanthes has every reason to write -anthus. The difference is grammatical. Those genera take a masculine Latin ending; Cheilanthes keeps a Greek form that comes out as eez in English. That faint eez at the end of the word is the only clue that it belongs to the other group.

An unstressed vowel, flattened until it gives the ear no information.