Geastrum saccatum is the earthstar most commonly featured in field guides--probably because it is widely distributed and common. It is a small but beautiful mushroom that features a round spore case sitting atop a star with 4-9 arms. Several species of Geastrum are very similar, however, so precise identification should rely on the following features:

Small size (2-5 cm across when arms expand); A spore case that sits directly on the arms, as though in a bowl (without a pedestal); A hole in the spore case that is not prominently lined; A circular ridge or depression around the hole;