Both are conventionally united as infraclasses within the subclass Theria (Parker and Haswell, 1897), which contains all living mammals except monotremes.
The name Neocephalopoda was first published (in Lehmann & Hillmer, 1980) as an Infraclass, which (in a reversal of the usual Linnean hierarchy) included the Subclasses Bactritoidea, Ammonoidea, and Coleoidea.
Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives