umbilicate

Sporangia orbicular, very much depressed, often umbilicate above and concave underneath, stipitate or sessile, gregarious, with the margins of the sporangia sometimes confluent.

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

Sporangium globose, the base slightly flattened or umbilicate, stipitate; the wall a thin membrane, covered with small scales of lime, yellow or greenish-yellow, breaking up and falling away at maturity.

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

Cap umbilicate, slightly depressed in the centre, b; gills decurrent, i. e., running down the stem.

From Student's Hand-book of Mushrooms of America, Edible and Poisonous by Thomas Taylor

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

The author says in his further description that the form affine is less definitely umbilicate, has white stems, etc.; allantoid, one would now say.

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