The Evolution of Inoceramus in the Cretaceous Period

Abstract

Introductory Remarks Although the genus Inoceramus is found in deposits as early as the Lias, yet it is represented by comparatively few species until the close of the Lower Cretaceous Period, after which it underwent rapid evolution so that many species and varieties were developed; but, so far as we know, none of these survived the Cretaceous Period or left descendants. From the evidence which has been furnish0ed by a study of the variation of the species, their zonal distribution, morphological resemblances and development, and from the discovery of connecting links, it now seems possible to recognize the main lines of evolution in the Inocerami of the Cretaceous Period. The species of Inoceramus found in the Gault, the Upper Greensand, and the Chalk appear to have originated from two stocks which occur in the Lower Greensand (Aptian), one being I. salomoni d'Orb., the other belonging to the type of I. neocomiensis d'Orb. and I. ewaldi Schlüt. I. Species connected with Inoceramus salomoni . We may first deal with the species which are connected with I. salomoni , namely:— I. concentricus, I. concentricus var. subsulcatus, I. sulcatus, I. tennis , and I. etheridgei , of which the distribution and relationship are shown in the accompanying diagram (A). I. salomoni d'Orb. (figs. 1–4, p. 3) is found in the Folkestone Beds (Lower Greensand) and in the Mammillatum Bed at the base of the Gault. The left valve is characterized by its subquadrate outline, the obliquely triangular hinge-area, the large anterior area bounded by a rounded ridge, the