. Bulletin of the British Museum (Natural History). Zoology . SHELL STRUCTURE OF THE BIVALVIA. Fig. i8. Block diagram of part of the calcitic prismatic layer of Atrina vexillum. x 150. PRISMATIC STRUCTURE Plate 7, figs. 1, 2, 4, 5, 6 ; Pis. 8, 9, 10, 11, 12, 13 ; PI. 26, fig. 5 ; text-figs. 2, 17, 18, 22, 75. Together with nacre, prismatic structures are very well known, and descriptions of prisms have been given by many authors, notably Carpenter (1844, 1848), Rose (1858), Biederman (1901), Schmidt (1922, 1924, 1925), B0ggild (1930) and Haas (1929-1935). Two main types of prism a
. Bulletin of the British Museum (Natural History). Zoology . SHELL STRUCTURE OF THE BIVALVIA. Fig. i8. Block diagram of part of the calcitic prismatic layer of Atrina vexillum. x 150. PRISMATIC STRUCTURE Plate 7, figs. 1, 2, 4, 5, 6 ; Pis. 8, 9, 10, 11, 12, 13 ; PI. 26, fig. 5 ; text-figs. 2, 17, 18, 22, 75. Together with nacre, prismatic structures are very well known, and descriptions of prisms have been given by many authors, notably Carpenter (1844, 1848), Rose (1858), Biederman (1901), Schmidt (1922, 1924, 1925), B0ggild (1930) and Haas (1929-1935). Two main types of prism are recognised here, simple prisms and composite prisms, and these are discussed, in turn, below. Prismatic layers of both types usually form the outer layers of shells in families where they occur. Simple prisms may be aragonite or calcite, but composite prisms are always aragonite. Many earlier workers have referred to prismatic layers in the inner and middle shell layers of bivalves (the dependent prismatic layer of B0ggild). We regard such layers as myostracal bands. Simple prismatic structure This is the best known type of prismatic structure and can occur as either aragonite (text-figs. 17 ; Plate 7, figs. 2, 4, 5 ; PI. 10, PI. n, figs. 1, 2) or calcite (text-figs. 18 ; Plate 7, figs. 1,6; PI. 9), with only minor differences in form ; the most consistent such difference is that calcite prisms have transverse striations (PI. 7, figs. 1, 6) while aragonite prisms have longitudinal, diverging striations as well as less con-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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