Hylonomus aciedentatus (Dawson 1860) lived ~310 mya and was slightly smaller than its its sister, Coelostegus. Both were derived from a sister to Paleothyris and preceded Protorothyris.
In the skull the temporal ledge is gone. Both Coelostegus and Hylonomus do not have the descending cranium seen in Paleothyris and Protorothyris. The mandible was concave dorsally. A likely insect-eater.
As in Coelostegus, fewer than 31 presacral vertebrae are present. The cervical neural spines are taller than the centra. Rod-like garstralia are present. the tail was long and slender, perhaps longer than the presacrals.
The scapulocoracoid was not well ossified.
Although the ilium was taller than long, the posterior process was retained. Pedal 4.1 was the widest of the proximal phalanges. Pedal digit V was longer. Metatarsals II and III aligned with p1.1.