Bactritids are distinguished from the more primitive nautiloids by the small size and globular shape of the protoconch, the so-called embryonic shell.

[2][3] The Parabactritidae[2] are characterized by orthoconic and breviconic shells with a large apical angle (greater than about 10°) and septal necks that are vary from orthochoanitic or suborthochoanitic to cyrtochoanitic.

Bactroceras (Eobactrites), an early middle-Ordovician cephalopod, is considered as a true bactritid by some paleontologists,[2] because of its spherical apex and ventral siphuncle.

For instance, the first chamber of Bactroceras resembles that of other Ordovician orthocerids, such as Archigeisonoceras and Hedstroemoceras: it is about 10 mm in diameter, and is short, forming a spherical cap.

Moreover, a large stratigraphic gap of nearly 50 million years occurs between Bactroceras and the next orthocones with a spherical apex and ventral siphuncle.