Abstract: The ammonite family Acanthoceratidae de Grossouvre is represented in the Western Interior region by many genera. Species and subspecies of Calycoceras, Acanthoceras, Cunningtoniceras, Plesiacanthoceras, and Protacanthoceras are important guide fossils to narrow zones of middle Cenomanian age in southeastern Montana, northeastern Wyoming, western South Dakota, and central Kansas. In this region, the ammonite zonation through most of the middle Cenomanian seems to be, from oldest to youngest, a zone of Acanthoceras muldoonense Cobban and Scott, a zone of Acanthoceras bellense Adkins and Calycoceras leonense (Adkins), a zone of Cunningtoniceras amphibolum (Morrow) with subzones of C. amphibolum amphibolum and C. amphibolum fallense (n. subsp.), and a zone of Plesiacanthoceras wyomingense (Reagan). In this report, Acanthoceras muldoonense, A. bellense, and Calycoceras leonense are described from the northern part of the Western Interior for the first time. Cunningtoniceras amphibolum, originally assigned to Acanthoceras, is described in more detail and divided into the nominate subspecies and the new subspecies C. amphibolum fallense. The earliest whorls of Plesiacanthoceras wyomingense, previously unknown, are described as well as all the later growth stages. The small genus, Protacanthoceras, is represented by several new species and one new subspecies. Protacanthoceras fisheri is found in the zone of Acanthoceras …