Ptenoceras was a nautiloid, a cephalopod related to modern-day nautiluses. The shell of Ptenoceras was loosely coiled, unlike the straight, conical shells of earlier nautiloids.
Ptenoceras is also one of the few extinct cephalopods where fossils have preserved clues about the markings on the shell; an examination of some fossilised Ptenoceras shells show that they had dark, banded markings on a pale background, similar to the markings seen on the shells of modern-day Nautilus.