UCMP MYSTERY FOSSIL #53 is . . .
a test of the foraminiferan Fissurina bandyi

Foraminifera, single-celled protists with shells (tests), have been around since the Cambrian Period. These primarily marine organisms are found all around the globe and are an important part of the marine food chain. Since different species of forams are found in different environments, their fossils are very helpful in determining past environments. The chemistry of fossil foram shells are also helpful in determining past water chemistry and temperature.

This particular foram test is from the Late Cretaceous Rosario Formation, San Diego County, California. To learn more about foraminifera, look here.