or many decades I have pondered over the identity of two black, crustose "lichens" covering monzogranite rocks and boulders in the Palomar College Arboretum. Many years ago, I attempted to key these "lichens" down to species with two very old, classic references. I checked the names with the University of California Checklist of California Lichens; however, I never felt comfortable with the identity of the following black coating on boulders.

1. Fink, B. 1935. The Lichen Flora of the United States. University of Michigan Press. 424 p.

2. Hasse, H.E. 1913. "The Lichen Flora of California." Contributions From the United States National Herbarium 17 (1): 1-132.

3. University of California Checklist Of California Lichens by Shirley C. Tucker & Bruce D. Ryan

Dark coating on rocks & boulders in the Palomar College Arboretum. I once thought it was the lichen Buellia pullata; however, I now think this may be a non-lichenized rock fungus. My conclusion explains the lack of characteristic lichen structures, such as cup-shaped apothecia of ascolichens. Under microscopic examination (400x) with compound microscope it lacks green, photosynthetic algal cells (photobionts), hymenial layer and paraphyses. It does have brown, 2-celled spores mixed with numerous colorless fungal cells. See following images taken 7 Mar. 2022.

Another crustose rock lichen with a dark brown or black thallus grows on rocks and boulders in the Palomar College Arboretum. This is a pyrenocarpous lichen with perithecia rather than apothecia. The perithecia are embedded in the thallus and are very small. Younger perithecia are almost completely embedded in the thallus. In older perithecia, the emergent portion of the perithecium is conical-spherical, 0.2-0.4 mm in diameter. The spores are colorless and simple (non-septate), about 20 µm long and 8 per ascus. The paraphyses break down or gelatinize (deliquesce) and are not visible when the spores are mature. The thallus is areolate, cracked into irregular sections (areoles) like dried, cracked mud. Another species in San Diego County, V. viridula, is very similar except the cracks between the areoles are wider. In the latter species the areoles are more distinctly separated from each other. In addition, the thallus of V. viridula appears greenish-brown when wet. V. nigrescens has an enormous worldwide distribution, including North America, Europe, Asia and northern Africa. Aquatic species of Verrucaria grow on boulders along streams and waterfalls, and the marine V. maura grows on rocks of the wave battered intertidal zone.