It seems to me almost useless to send the fragments of the shell to the British Museum to be named, more especially as the umbo has been lost.
"More Letters of Charles Darwin Volume II" by Charles Darwin
The central umbo or crest posteriorly is a marked feature.
"Narrative Of The Voyage Of H.M.S. Rattlesnake, Commanded By The Late Captain Owen Stanley, R.N., F.R.S. Etc. During The Years 1846-1850. Including Discoveries And Surveys In New Guinea, The Louisiade Archipelago, Etc. To Which Is Added The Account Of Mr. E.B. Kennedy's Expedition For The Exploration Of The Cape York Peninsula. By John Macgillivray, F.R.G.S. Naturalist To The Expedition. In Two Volumes. Volume 1." by John MacGillivray
They are sometimes bell-shaped and then flattened, often with a mound or umbo.
"Among the Mushrooms" by Ellen M. Dallas and Caroline A. Burgin
The =pileus= becomes convex, and expanded or nearly flat, and often with an elevation or umbo in the center.
"Studies of American Fungi. Mushrooms, Edible, Poisonous, etc." by George Francis Atkinson
The position of the umbo on the apophysis is the basis of Koehne's subdivision of the section Haploxylon.
"The Genus Pinus" by George Russell Shaw
Many plants have a marked umbo on the top of the cap and the rim of the cap may be slightly turned up.
"The Mushroom, Edible and Otherwise" by M. E. Hard
A ridge, generally runs from the umbo to the upper point.
"A Monograph on the Sub-class Cirripedia (Volume 1 of 2)" by Charles Darwin
P. 4-6 cm., umbo very strong, ochraceous, scaly; g. white; s. 9-15 cm.
"European Fungus Flora: Agaricaceae" by George Massee
The umbo is the oldest part of the valve.
"Elementary Zoology, Second Edition" by Vernon L. Kellogg