The original spelling is correct, althought it has frequently been altered to "hamiltonhoytae".
Reppanhagen mantaines this species (althought the plant he pictures does not have a purple flower), and adds the variety below.
Bravo in her latest work reduces it beneath M.obscura.
Hunt mantains his referral in previous pubblications to synonymy with M.gigantea, in spite of its purple flower, interpreting it "as a purple-flowered variant in the M.petterssonii - M.gigantea group."
It is significant that Craig described a yellowish-brown flowered variety (v. flavaflora), said to have the same distribution as the type, quoted as San Moran.
Referred to Mammillaria petterssonii.
Mammillaria - John Pilbeam pag.328