Slimonids were members of the superfamily Pterygotioidea and the family most closely related to the derived pterygotid eurypterids, which are famous for their cheliceral claws and great size.

Many characteristics of the Slimonidae, such as their flattened and expanded telsons (the posteriormost division of their bodies), support a close relationship between the two groups.

Slimonids are defined as pterygotioid eurypterids with swimming legs similar to those of the type genus, Slimonia, and the second to fifth pair of appendages being non-spiniferous.

The family contains only two genera, the almost completely known Slimonia and Salteropterus, which is known only from the telson and the metastoma (a large plate part of the abdomen).

[1] Like all other chelicerates, and other arthropods in general, slimonid eurypterids possessed segmented bodies and jointed appendages (limbs) covered in a cuticle composed of proteins and chitin.

[4] Though Slimonia itself is very well known, the other genus of the family, Salteropterus, is less well known with its fossils only preserving the telson and the metastoma (a large plate part of the abdomen).

[5] Though they were closely related, the Slimonidae had small and non-developed chelicerae (frontal appendages) in comparison to the Pterygotidae, which possessed well-developed and powerful cheliceral claws.

A specimen of Slimonia acuminata from the Patrick Burn Formation of Scotland preserves a complete and articulated series of telsonal, postabdominal and preabdominal segments.

[3] However, this theory has been proven to be erroneous as the fossil specimen in question was a molt, rather than an actual carcass, and did show apparent signs of disarticulation.

The flattened portion is trigonal and smaller than that of Slimonia but the telson spike is far longer, forming something akin to a stem with knobs running alongside it and ending in a tri-lobed organ unseen in any other eurypterid.