ABSTRACT
The standard shell structure of rhynchonellate brachiopods, as typified by the living rhynchonellide Notosaria consists of a primary layer of growth-banded calcite succeeded by a secondary layer of fibres sheathed in interconnected glycoproteinaceous membranes. The extreme variations include the atrophy of the secondary layer in living thecideidines, like Thecidellina, and the development of a tertiary layer of ‘prismatic’ calcite in the terebratulide, Liothyrella. An ultrastructural comparison of the three kinds of shells shows that they are virtually identical in correlated layers. The basic calcitic unit is a granule, ca. 15 nm in diameter, aggregated as rhombohedral tablets and spherules that are probably ‘glued’ together by a polysaccharide to form laminae. In the primary and tertiary layers, laminae are nearly horizontal. In secondary layers, laminae are variably inclined but normally retain the same crystallographic alignment not only in contiguous fibres but also with those of the primary and tertiary layers. Laminae of fibres commonly aggregate as rhombohedral laths. Such laths cleave parallel with rhombohedral angles subtended by the proximal boundaries of terminal faces.
