Third, this white mudstone, about twenty feet in thickness, and divided into two varieties (C and D), both closely resembling the lower, fine- grained, more unctuous and compact kind at Port Desire; and, as at that place, including much selenite. Fourth, a fossiliferous mass, divided into three main beds, of which the uppermost is thin, and consists of ferruginous sandstone, with many shells of the great oyster and Pecten Paranensis; the middle bed (E) is a yellowish earthy sandstone abounding with Scutellae; and the lowest bed (F) is an indurated, greenish, sandy clay, including large concretions of calcareous sandstone, many shells of the great oyster, and in parts almost made up of fragments of Balanidae. Out of these three beds, I procured the following twelve species, of which the two first were exceedingly numerous in individuals, as were the Terebratulae and Turritellae in certain layers:— 1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe, and whole coast of Patagonia). 2. Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (St. Fe, S. Josef, Port Desire). 3. Pecten centralis, G.B. Sowerby (also at Port Desire and S. Cruz). 4. Pecten geminatus, G.B. Sowerby. 5. Terebratula Patagonica, G.B. Sowerby (also S. Josef). 6. Struthiolaria ornata, G.B. Sowerby (also S. Cruz). 7. Fusus Patagonicus, G.B. Sowerby. 8. Fusus Noachinus, G.B. Sowerby. 9. Scalaria rugulosa, G.B. Sowerby. 10. Turritella ambulacrum, G.B. Sowerby (also S. Cruz). 11. Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat. 12. Balanus varians, G.B. Sowerby. 13. Scutella, differing from the species from Nuevo Gulf.