Shell-structure of the Brachiopod Lacazella mediterraneum (Risso)

Abstract

THE continued use of Beecher's ordinal classification¹ of the Brachiopoda despite its inconsistencies² is mainly due to his fascinating interpretation of Kovalevskii's observations³ on the development of the recent thecideid Lacazella. Beecher concluded that the pseudodeltidium was secreted not by the mantle fold of the thoracic segment but by the caudal segment, and that only later was it ankylosed to the pedicle valve. Further, the pseudodeltidium appeared to be identical with a similar structure invariably found in the extinct strophomenoids (the central group of his order Protremata), a comparison that led him to regard the thecideids as derivatives of that stock. In other articulate brachiopods (the Telotremata of Beecher), the delthyrium is usually constricted by deltidial plates, sometimes fused into a single unit, which are undoubtedly secreted by extensions of the mantle. Thus on the assumption that the pseudodeltidium was a ‘third shell’, Beecher was able to draw a fundamental distinction between the two groups. Substance was added to the supposed alien nature of the pseudodeltidium by his belief that in protrematous brachiopods the pseudodeltidium was always impunctate, even when the valves were punctate as in the strophomenoids.