Fruiting body 3.0-6.0 (9.0) cm tall, 3.0-4.0 cm broad, top-shaped, stipitate, broadly cupulate at maturity; margin upturned to incurved, sometimes wavy; exterior wrinkled to furrowed, covered by a fine, dark-grey tomentum, blackening where handled; interior or hymenial surface black, more or less glabrous, slightly furrowed; context a blackish gel filling the tapered base and the cup wall, odor not distinctive; taste mild.
Spores 23-34 x 9-14 µm, smooth, thin-walled, hyaline, ellipsoid, some inequilateral, appearing sausage-shaped; one to three guttules present at maturity; spore print not seen.
Solitary, in small groups, occasionally clustered, on rotting or buried wood in coastal forests, possibly also in the Sierra Nevada; fruiting from mid-winter to spring; uncommon.
This rubbery, black cup fungus with a wrinkled "stipe" is easily overlooked because of its preference for dark woods and rotting wood. Like many Ascomycetes, it fruits from mid to late winter. A smaller relative, Sarcosoma latahense, has a less gelatinous context at maturity, but is otherwise similar, and requires a microscope to accurately determine. Several other black cup fungi resemble Sarcosoma mexicanum. Most common is Bulgaria inquinans which often fruits in large numbers on hardwood logs. Its blackish-brown rubbery cups have a furfuraceous rather than wrinkled exterior; Plectania melastoma, also lignicolous, is a thin-fleshed, sub-stipitate black cup that has a persistently incurved margin often tinged orange from granules encrusting the external hyphae; Plectania milleri differs from the above in possessing a toothed cup margin and lacks encrusting orange granules. In montane regions, two additional black cups are found in the spring. Plectania nannfeldtii, a snowbank species, forms black goblet-shaped cups on thin, slender stipes attached to rotting wood. Less conspicuous and infrequent is Pseudoplectania nigrella, a small, sessile to substipipitate cup with a hairy exterior.