The aïstopod family Phlegethontiidae is restudied based on new specimens from Pit 11 of Mazon Creek, Illinois, and the coal shales of Nýřany, Czech Republic, as well as most available specimens from North America. Phlegethontiids have highly fenestrate skulls, with orbits placed just anterior their skull's mid point. Dermal skull bones are greatly reduced in number and limited in extent, whereas the endochondral braincase is hyperossified. The frontals are fused medially and enclose the parietal foramen and anterior sagittal crest. As in most other aïstopods, the quadrate, pterygoid, and epipterygoid are fused into a composite bone, the palatoquadrate complex. Details of cranial anatomy contradict a previous model of cranial kinesis by severely limiting the skull's potential mobility. Remnants of the pectoral girdle are present, perhaps due to the presence of an operculum–opercularis-like connection to the stapes. No remnants of the pelvis are present.

Three species are recognised within the family. Phlegethontia linearis has short anterior vertebrae, high neural spines on at least the anterior four vertebrae, and vertebrae number between 230–250 in total. Phlegethontia longissima has low neural spines throughout the column, anterior vertebrae that are twice as long as P. linearis, and only 200–210 total vertebrae. Sillerpeton permianum, known from a single braincase and an unassociated string of vertebrae, is distinguished from Phlegethontia by the retention of a separate foramen for the passage of the occulomotor nerve. Phlegethontia “phanerhalpa” is a tiny braincase fragment that differs from the other species of Phlegethontia only in the placement of the jugular foramen relative to the centre of the foramen magnum. This is probably a size-related feature, and P. “phanerhalpa” is considered a nomen dubium.