Through the kindness of Miss Errington, of Bear Valley, Mariposa County, I have had the opportunity of examining a series of fossils, discovered by that lady on the Mariposa Estate. The shells appear to be of Jurassic age. The genera so far recognized, are Belemnites, Nucula or Leda, Lima, Pecten, and Pholadomya. The following are sufficiently perfect for description. Fuller descriptions and figures will be published hereafter in the Geological Report of the State. LIMA, Brug. _L. Erringtoni_ G. Shell very oblique, sides subparallel; beaks small; anterior end prominently rounded, sloping with a broad curve below to the base; posterior side nearly straight, continuing upwards in a long narrow ear; anterior ear obsolete (?). Surface marked by strong concentric undulations, crossed on the upper and posterior portions by fine radiating lines. Length, 2.25 inches; width, 1 inch. The specimens are all very much compressed, and may be somewhat distorted in form, though the outlines are very nearly uniform in all of the specimens. PHOLADOMYA, Sow. _P. orbiculata_ G. Compressed, sub-circular; beaks rather prominent, nearly central; anterior end and base regularly rounded; posterior end slightly produced, more prominent below than above. Surface ornamented by concentric ribs, irregular in size, crossed by undulating, radiating lines on the anterior half of the shell. These lines become gradually obsolete and disappear on or about the middle of all of the specimens, although more perfect examples might show them continuing further. Height, from a slightly distorted specimen, 1.1 inch; width, 1.3 inch. Like the preceding species, all of the specimens have suffered considerably by compression; and had we perfect specimens, the shell would be found to be quite convex. BELEMNITES. _B. Pacificus_ G. Long, slender, tapering very gradually and with a slight convexity to the tip. Section, sub-elliptical; alveolus deep and narrow. A specimen 3 inches long, measures .35 in. in diameter in the middle. One cast has been found with a diameter of .9 inch at the broadest part. I first discovered this species, as imperfect casts, in the slates near Spanish Flat, El Dorado County. Since then, numerous fragments and casts have been found at Mariposa, by Mr. C. R. King and Miss Errington. Communication on the San Luis Obispo Quicksilver Fossils. BY WM. M. GABB. Mr. Attwood has recently presented to the collection of the State Geological Survey, a series of Fossils collected by himself, in the formation in which the San Luis Obispo cinnabar deposits occur.