Ammonius is a bowl-shaped lunar impact crater with a slightly raised rim. It is located on the floor of the walled plain Ptolemaeus, about 30 kilometers northeast of the crater mid-point. In the past this crater was identified as Ptolemaeus A, before being named by the IAU.

Just to the north on the lava-flooded floor of Ptolemaeus is a relatively prominent "ghost" crater: the discernible buried rim of a pre-existing crater. The diameter of this ghost crater is nearly double that of Ammonius, and is currently identified as Ptolemaeus B.

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