umbilicate

Sporangia globose, sometimes a little depressed, with the base umbilicate; the wall steel-blue with tints of purple and violet, quite persistent, rupturing irregularly.

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

Sometimes they are umbilicate below, so that a vertical section would be obcordate.

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 hemispherical, more or less depressed, the base profoundly umbilicate; the wall firm, rugulose, dark-colored and nearly opaque, with a mealy coat of stellate crystals of lime, rupturing irregularly.

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

Sporangium depressed-globose, the base umbilicate, stipitate; the wall a thin, violaceous membrane, brownish at the base, with a thin coat of small, white scales and minute granules of lime.

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

Sporangium depressed-globose, the base umbilicate, stipitate, rarely sessile and plasmodiocarp; the wall a dark-colored membrane with a thin layer of stellate crystals of lime, breaking up gradually and falling away.

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