acervulus

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A tiny cushionlike or blisterlike structure produced by certain fungi on a plant host and consisting of a mass of hyphae-bearing asexually produced spores.

Century Dictionary and Cyclopedia

1. n. 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).
2. n. 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.

Wiktionary

1. n. calcified concretions near the pineal gland; brain sand

WordNet 3.0

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