The only remains of the genus consist of a crushed skull, shoulder girdle bones, and scales, which were similar to microsaurian elements originally referred to Asaphestera.

Boii can be characterized by its heavily sculptured skull, thin ventral plate of the clavicles, and a larger number of fangs on the roof of the mouth.

[4] The specimen now designated as Boii crassidens was first described by Antonin Frič, one of the most notable paleontologists in the late 19th century region of Bohemia (now part of the Czech Republic).

The article, which was published in 1876 within volume 1875 of the journal, was a list of Carboniferous animals he and his associates recently discovered at gaskohle (bituminous coal) mines near the localities of Nýřany and Kounová.

A more elaborate description was published in 1883 as part of a personal monograph focusing solely on the creatures discovered at these mines.

This prompted Carroll to create a new genus name for the specimens previously considered to belong to "Sparodus" crassidens.

[4] The crushed skull was pressed between two plates of coal which preserved the outer impressions of bones on both the underside and upper side of the specimen.

The maxillary and premaxillary bones forming the edge of the snout contained many conical teeth, about 30 per each side of the upper jaw.

Large palatal fangs are also shared by Sparodus, explaining how Boii crassidens was once considered to be part of that genus.

A row of small pits run from the symphysis (chin) along the upper portion of the outer face of the bones.

A single arm bone was also present on the specimen, although paleontologists disagree whether it was a humerus (according to Frič, 1883)[3] or a radius or ulna (according to Carroll, 1966).

[3] Branchiosaurids, a group of small gilled temnospondyls, are now believed to be only distant relatives of Sparodus and Boii.

[4] However, as of 2020 Asaphestera has been recognized as a chimeric taxon, based on specimens of a potential eothyridid along with a newly-named microsaur, Steenerpeton.