UCMP MYSTERY FOSSIL #43 is . . .
the foraminiferan Elphidium granti

Foraminifera are single-celled protists with shells that are important in the study of past environments. They are found in all marine environments, from the intertidal to the deepest ocean trenches, and from the tropics to the poles, but species of foraminifera can be very particular about the environment where they live. Because of this, data such as species diversity, the relative numbers of planktonic and benthic species (planktic:benthic ratio), and the ratios of different shell types can say much about these past environments. In addition, the chemistry of the shell can tell us about the chemistry of the water in which it grew. To find out more about foraminifera, look here.