Foraminiferal tests have been studied in the electron-microscope using carbon replicas. Thin sections of the tests have been studied in transmitted and polarized light and phase microscopy. Miliolids show two inter-grading kinds of surficial ultrastructure; some areas are made of numerous oriented rhombs of calcite, presenting a pattern resembling a tile roof; other areas show randomly distributed rods of calcite. In phase microscopy, sections of the miliolid test present a furry appearance. Peneroplis planatus shows a more massive surficial ultrastructure, and presents a different appearance in polarized light, certain layers having oriented calcite crystals. Nodosaria affinis and Robulus midwayensis have perforate areas with about 50 pores per 100 square microns surface area, each pore being about ½ micron in diameter. Imperforate areas of these tests are made of relatively large crystals of calcite, up to 2 microns in diameter. Sieve plates have been found covering the ends of pore canals in R. midwayensis. Sphaeroidina bulloides and Bulimina marginata are strikingly similar in their ultrastructure. They have about 40 pores per 100 square microns surface area. The pores average ½ micron in diameter. Discorbis vesicularis has a rough surface, and only about 4 pores 2 microns in diameter per 100 square microns surface area. Elphidium macellum has a finely finished surface and 120 pores ½ micron in

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diameter per 100 square microns surface. Globigerina bulloides is roughly finished, and possesses only large conical pores spaced widely apart over the surface.

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