Known as BMNH In 60023, it is poorly preserved and its type locality is uncertain, although it might be the Kelly Den stream section near the village of Arbirlot.

Forming part of the family Diploaspididae, it was similar to Diploaspis and other Devonian genera, although it is speculated that Forfarella may have existed during the Silurian as well.

The details on its surface are not preserved with the exception of a tubercle in the left anterior corner of the carapace, only distinguished if it is placed under plane polarized light.

[4] In 1962, the British paleontologist Charles D. Waterston borrowed the still unregistered specimen, as well as four other fossils of juvenile eurypterids, to describe them.

However, Waterston would never publish his description, returning the fossil to the museum three years later, where it would be labeled with the same name given by the paleontologist.

Thus, Forfarella mitchelli remained as a nomen manuscriptum (a taxonomic name that appears in an informally published document).

[4] Despite the turbulent history of the fossil, the paleontologists Jason A. Dunlop, Lyall I. Anderson and Simon J. Braddy formally proposed and described Forfarella mitchelli for the first time in 1999 as a new genus of chasmataspidid.

[5][1] At the time Forfarella was described, members of Diploaspididae were defined as small chasmataspidids with a subrectangular or semicircular carapace, a tapering postabdomen and a short telson.

Regarding other chasmataspidids, Forfarella was considerably similar to Diploaspis casteri, although it had a longer postabdomen, perhaps because of a taphonomic distortion (that is, a defect product of the fossilization of the organism) of the specimen.

What differentiated Forfarella from the rest of the chasmataspidids was the dimensions of its body, its size, the shape of its carapace and the distinctive subtrapezoidal preabdomen.

Its lithology (the physical characteristics of the rocks of the fossil) is similar to that of the laminate fish-bearing beds in Tealing, a Scottish village near from the suspected type locality.

This type of chasmataspidids subsequently invaded freshwater habitats, maybe due to the Late Devonian extinction event.

This also suggests that there could be specimens from earlier times of other diploaspidids similar to Diploaspis, such as Achanarraspis or Forfarella, that have yet to be discovered (i.e. a "ghost" range).