umbilicate

Sporangium globose, the base sometimes flattened or slightly umbilicate, stipitate; the wall a thin, pellucid membrane, covered with small scales of lime, white, cream-colored, or sometimes pinkish, breaking up and falling away at maturity.

From The Myxomycetes of the Miami Valley, Ohio by A. P. (Andrew Price) Morgan

P. compressum                             ++ Sporangia typically globose, umbilicate below, connate, etc., strongly calcareous 39.

From The North American Slime-Moulds A Descriptive List of All Species of Myxomycetes Hitherto Reported from the Continent of North America, with Notes on Some Extra-Limital Species by Thomas H. (Thomas Huston) MacBride

Pileus fleshy at disc, thinner towards the margin, becoming deeply umbilicate or infundibuliform.

From European Fungus Flora: Agaricaceae by George Massee

The latter is usually very short, almost completely concealed in the concavity of the umbilicate sporangium.

From The North American Slime-Moulds A Descriptive List of All Species of Myxomycetes Hitherto Reported from the Continent of North America, with Notes on Some Extra-Limital Species by Thomas H. (Thomas Huston) MacBride

Sporangium depressed-globose, umbilicate at the apex, stipitate, cernuous, purplish-brown in color; the calyculus granulose within, occupying from one-fourth to one-third of the sporangium, the ribs united by firm, persistent fibers.

From The Myxomycetes of the Miami Valley, Ohio by A. P. (Andrew Price) Morgan