Remarkably well-preserved fossils of the udoteacean alga Eugonophyllvm occur in the Virgilian Holder Formation, Sacramento Mountains, south-central New Mexico in a 20-cm-thick packstone bed between two thick shale beds. Original aragonite is preserved in this bed as a felt-like mesh of needles in algal thalli, in mollusc shell fragments, in sponge walls, and in botryoidal and isopachous marine cements. The average Sr content of the algal aragonite (9091 ppm, n = 21, standard deviation = 1498) is comparable to reported values for modern Halimeda. Internal and cortical structures of Evgonophyllvm are exceptionally well preserved. In the basal, heavily calcified portion of the alga the medullary (interior) region of the Eugonophyllum thallus is composed of an aragonitic felt interspersed by small (20 μm diameter), parallel siphons