Like plectronocerids, their shells were compressed and slightly curved, containing numerous closely-spaced septa.

The connecting rings have a calciosiphonate structure, while the siphuncle is divided by narrow partitions known as diaphragms.

Protactinocerids were originally differentiated from plectronocerids based on their proportionally wider siphuncle.

[2] However, it is difficult to distinguish biological calcite deposition from later diagenetic infilling, so this trait is also dubious.

[4] Some authors have suggested merging Protactinocerida into Plectronocerida in light of the uncertainty in differentiating the two orders.