The Relation of the Valves to the Pedicle in the Strophomenid Brachiopods

Abstract

The relation of the valves to the pedicle is among the fundamental characters on the basis of which the phylum Brachiopoda has been divided into five orders (see Thomson, 1927). In a previous paper (Arber, 1939), I pointed out that, on this classification, the order Protremata cannot be regarded as a homogeneous group, since in the orthids the pedicle apparently emerged through an open delthyrium, while in the strophomenid Leptaena it emerged through an entirely separate foramen situated in the ventral valve. Schuchert in 1913 (p. 383) defined the whole superfamily Strophomenacea as being characterized by a pedicle which, when present, was small, and emerged at the apex of the valve. A pedicle-foramen is, however, by no means of universal occurrence, and its existence has also been often overlooked in systematic descriptions, so that, in spite of Schuchert's definition, some doubt has nevertheless remained as to whether the form of pedicle-opening found in Leptaena and a few closely allied genera is genuinely typical of the whole of the strophomenids, thus creating a developmental distinction between that group and the orthids. I have therefore examined as many as possible of the available specimens and descriptions of the strophomenids, with the results here recorded.