acervulus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small cushionlike structure produced by certain parasitic fungi, containing a mass of asexually produced spores.

from The Century Dictionary.

• noun The fruiting pustule of certain fungi, as Glæosporium and related genera, consisting of small dense masses of conidiophores and conidia formed beneath the epidermis of the host, which bursts and permits the escape of the conidia when they mature.
• noun In anatomy, a mass of calcareous gritty particles, consisting principally of earthy salts, found within and sometimes on the outside of the conarium or pineal body of the brain; brain-sand. Commonly called acervulus cerebri (acervulus of the brain).

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun calcified concretions near the pineal gland; brain sand

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun small asexual fruiting body resembling a cushion or blister consisting of a mat of hyphae that is produced on a host by some fungi