The Ammonites of the Green Ammonite Beds of Dorset

Abstract

I. Introduction. As on previous occasions when I have had the privilege of examining the ammonites of the Lower Lias of Charmouth, collected by Dr. W. D. Lang, I propose to discuss the identifications, given in his stratigraphical account; also to comment on the generic classification and the phylogenetic relationships of the ammonites. Of four families recorded, only one (the Liparoceratidæ) is really abundantly represented and important; but on account of the necessity for adequate illustration I am publishing a full discussion of the phylogeny of this family elsewhere. Here I may stress the fact that the V-shaped peripheral ribs so characteristic of the late capricorns first appear on the inner (not the outer) whorls of the early forms, a clear case of cænogenesis, as palæontologists understand it, and comparable to that of schlotheimids (Spath, 1924, p. 198); also, the fact that the restriction of the various ammonites to certain beds is fortuitous and peculiar to Dorset, having nothing to do with the real range of these fossils. That is to say, the discovery of an Amaltheus at a level 11 feet below the Lowest Tier enables us to fix the local base of the Margaritatus Zone, and it is convenient that, at least up to the present, Oistoceras has not been found to range higher than within three feet of the lowest Amaltheus . But where Oistoceras and Amaltheus overlap, or where even Prodactylioceras davœi passes up into the Margaritatus Zone, or where Liparoceras ( Becheiceras ) has its maximum in the