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Agathotoma candidissima, common name Cox's mangelia, is a species of sea snail, a marine gastropod mollusc in the family Mangeliidae.
The length of the shell varies between 4 mm and 11 mm.
The white oblong, turreted shell contains 7 or 8 whorls (the nucleus of the specimen has broken off). The axial sculpture consists of 7 to 8 ribs. The manner in which some of the ribs on the body whorl turn suddenly at right angles to the base of the columella is very remarkable. The penultimate and the three or four preceding ribs exhibit this character. At the upper ends, a little below the suture, the ribs are faintly angled ; and the dots on the base in the upper whorls and on the middle and lower part in the body whorl are so faint that they might be easily overlooked. There are also a few faint dots between the ribs, just below the suture. The aperture is elongate-ovate and covers about 3/7 of the total length of the shell. The outer lip is incrassate and on top slightly insinuate. The siphonal canal is short. The base of the shell is truncated.
This species occurs in the Caribbean Sea, the Gulf of Mexico and the Lesser Antilles. Fossils have been found from the Early Pleistocene to the Middle Pleistocene in Southern Florida.
