Several Geastrum and Astraeus earthstars look similar. However, Myriostoma coliforme is the only earthstar that has many mouths on the inner peridium, so this is the most distinguishing characteristic to use to separate it from the others. It also is the only earthstar that features several short stalks connecting its inner and outer peridiums (other earthstars sometimes have one stalk, but never several).

See Tom Volk’s page on earthstars.

When young, before the outer peridium has split open, earthstars can resemble puffballs. Myriostoma features 3 peridial layers (the mesoperidium is often inseparable from the exoperidium, but is usually distinct), where as true puffballs only have 2 peridial layers. Also, true puffballs feature basidia arranged in a hymenium, and their spores are usually smooth.

See Tom Volk’s page on puffballs.