Dorsal surface convex, becoming somewhat keel-shaped towards the apex, which is strongly and obliquely deflected so as to make the right border nearly a straight line, while the expansion on the left projects nearly as far back as the apex, at an obtuse angle. Structure corneous, with strong concentric lines of growth, and faint radiating striæ. Color dark brown, opaque; inner surface shining and purplish, the plate white towards the edge, and in some specimens showing a thickened, white semicircle continuous with its margin across the arch of the shell. Length about sixteen one-hundredths, breadth eight one-hundredths, and height six one-hundredths of an English inch. More than fifty specimens were found on water plants in clear stagnant ponds, two or more often sticking on the back of a larger one. The discovery of this little shell in California is of great interest, the only species hitherto known being found in Cuba. The generic characters of this shell are strictly parallel with that species, while those mentioned as specific easily distinguish it. The Cuban shell is more elongated, regularly oval, the apex projecting considerably beyond the margin of the aperture, which is not obliquely expanded posteriorly. Its size is about one-fifth larger than that of ours. According to Bourguignat, the young shell is a simple obtuse cone, with a semicircular aperture formed by the edge of the shelf, and the thickened dorsal margin; but as it grows, the animal changes the form of the aperture until the opening beneath the shelf becomes like the small end of a broad funnel, which in some of our specimens is still shown by the white semicircular ring. The shell much resembles that of the marine _Crypta_ (_Crepidula_), and also _Navicella_ of tropical estuaries; but the animal is quite different in the Cuban species, and will undoubtedly prove so in the Californian. Mr. Hanks mentioned that he had collected about two hundred specimens of minerals for the Academy in Owen’s Lake Valley, and that there were also some bones with them from a well thirty feet deep, presented by Mr. H. M. McCormick; all of which would be forwarded to San Francisco as soon as possible. REGULAR MEETING, MAY 4TH, 1863. President in the Chair. Present, eleven members. Donations to the Cabinet were received as follows: A collection of pine cones, from H. G. Bloomer. A specimen of rock, containing cretaceous fossils, from the vicinity of Fort Tejon (?), by E. T. Schenck. Two specimens of _Monocentris Japonicus_ Cuv. from Dr. Ayres. Donations to the Library: