Triangulate monograptids from the monograptus gregarius zone (lower llandovery) of the rheidol gorge (cardiganshire)

Abstract

A large collection of well-preserved specimens is used as a basis for a systematic study of Monograptus ‘triangulatus’ and its allies (Monograptids with non-overlapping triangular thecae, defined as Group V I in the Monograph of British Graptolites). Critical synonymies, diagnoses, descriptions and resemblances with other species are given, and three species an d six varieties are new. P articular attention is paid to two aspects of the work : (1) The type of preservation and the changes undergone by thecae on compression. M any of the specimens are pyritized, in full relief, an d from these, models were made of the shapes of uncompressed thecae which helped in interpreting the compressed ones. This enables revised descriptions of some species to be given. (2) The sequence of forms in the succession. Collection was made from definite narrow horizons and so tentative schemes of evolution could be drawn up. In this connexion some study has been made of Monograptus revolutus and related forms. There seems little doubt that Rastrites longispinus evolved from Monograptus triangulatus' and a form intermediate in both morphology and order of appearance has been found.