Separate groups of strata are often characterised by distinct species of nummulite; thus the beds between the lower Miocene and the lower Eocene may be divided into three sections, distinguished by three different species of nummulites, N. variolaria in the upper, N. laevigata in the middle, and N. planulata in the lower beds.
"The Student's Elements of Geology"
Sir Charles Lyell
The opinion of many observers, that the Nummulitic formation belongs partly to the cretaceous era, seems chiefly to have arisen from confounding an allied genus, Orbitoides, with the true nummulite.
"The Student's Elements of Geology"
Sir Charles Lyell
Dr. Carpenter and Professor T. Rupert Jones have since confirmed this opinion, comparing the structure to that of the well-known nummulite.
"The Student's Elements of Geology"
Sir Charles Lyell