Rotulidae

The generalized rotulid has a circular to oval-shaped test, and indentations starting along the posterior edge. In Heliophora, the indentations may remain restricted to the posterior edge, or they may reach to the anterior edges of the test. Depending on the individual, the indentations may be very shallow, or very deep, forming very long "fingers," or digits. While rotulids are very distinctive in appearance, they are also highly morphic, with a tremendous diversity seen in individual specimens. That the digits are very fragile, and prone to breaking off and regenerating only adds to individual variations.

In the genus Heliophora, the test is extremely variable. Depending on the individual, indentations may be shallow or deep, and may be restricted to the posterior edge, or may be found along the entire margin of the test.

In the genus Rotula, the test has up to eleven indentations along the posterior edge, forming up to twelve digits. The primary way to distinguish Rotula from Heliophora is that adult specimens of Rotula have up to four holes, or lunules, set into the anterior portion of the test.

In the extinct genus Rotuloidea, the test is oval-shaped, and comes to a blunt point at the anterior end of the test. There are nine, very shallow indentations along the posterior edge of the test. The tests of this genus are extremely similar to those of Heliophora, especially of young specimens, sometimes to the point of confusion.