In a recent examination of a considerable suite of specimens from the Western Interior of the United States belonging to the Cretaceous ammonite genus
Placenticeras Meek the writer made comparisons of the species from the Interior with those from the Cretaceous of the Pacific coast ordinarily designated
Placenticeras pacificum J. P. Smith, P. calijornicum Anderson, and P. sanctaemonicae Waring. The two groups of species differ widely in their characteristics,
though each is a fairly homogeneous assemblage in itself. It was the writer's intention to propose a new name for the group of P. pacijicum - indeed, one was
proposed and in type, when the publication by Spathof the name Metaplacenticeras met the need adequately, and that name is of course used here. Spath's name is based on the citation of Smith's original figure and a notation that Metaplacenticeras differs from Placenticeras in its falcoid ribbing and distinct suture line. Inasmuch as there has been some divergence of opinion as to the scope and probable derivation of the genus Placenticeras, based on the study by different writers of members of one or the other of the two groups, the writer has thought it worth while to record his observations and deductions and to give a fuller account of the new genus.