XXXIV.—Studies in the Ammonites of the Family Echioceratidæ

Abstract

The ammonites dealt with in this paper belong to the family Echioceratidæ (Buckman, T.A., 1913), a varied group of slender, generally evolute forms, usually ornamented by ribs but very rarely tuberculate. In many members of the family the periphery has only a feeble keel, but in some a sharp keel, with or without sulci, may be present.Ammonites of this family have most frequently been recorded as Ammonites raricostatus, and the raricostate species are by far the best known. The species at present classified with the Echioceratidæ, however, can be roughly grouped, with reference to their form only, as follows :—1. Raricostate forms, with strong widely spaced ribs and feeble carina, e.g. A. raricostatoides, A. zieteni.2. Forms with closely spaced ribs, with carina absent or feeble, e.g. A. viticola, A. prorsum.3. Forms with elevated whorls, bearing regular ribs of moderate strength and with carina and sulci, e.g. A. aplanatum.4. Forms becoming smooth on the last whorls, which are thin, with an acute keel and obsolescent sulci, e.g. A. macdonnellii.