The foraminifera Ammonia beccarii (Linné) has a trochoidal test. The morphology of the spiral side of the test, which shows a series of chambers grown during the life of the animal, can be expressed explicitly by two equations associated with six constants. One of the equations represents the constant expansional rate of the spiral whorl, and the other represents the sigmordal growth of chamber size. When these equations and constants are plotted, the result is a graphic simulation of the projected morphology of the animal. Similarity of the simulated morphology to that of the projected specimen supports the validity of the mathematic formulation of the morphology. Consequently, we conclude that the equations and constants not only describe the morphology of the animal but also offer bases for understanding its growth characteristics.