. Bulletin of the British Museum (Natural History). SHELL STRUCTURE AND MINERALOGY OF BIVALVIA 271 is made up of further smaller vmits about 0-5 [x in diameter and diverging in a feathery manner from the central axis of the larger crystallites (Plate 7, fig. 4). The outer layer is separated by various degrees of distinctiveness from the under- Ijdng crossed-lamellar layer which has concentrically aligned primary lamels (Plate 7, fig. i). This middle layer changes to homogeneous structure when traced towards the shell interior. In some species the distinctly crossed-lamellar portion of the laye
. Bulletin of the British Museum (Natural History). SHELL STRUCTURE AND MINERALOGY OF BIVALVIA 271 is made up of further smaller vmits about 0-5 [x in diameter and diverging in a feathery manner from the central axis of the larger crystallites (Plate 7, fig. 4). The outer layer is separated by various degrees of distinctiveness from the under- Ijdng crossed-lamellar layer which has concentrically aligned primary lamels (Plate 7, fig. i). This middle layer changes to homogeneous structure when traced towards the shell interior. In some species the distinctly crossed-lamellar portion of the layer is almost entirely suppressed (Plate 7, fig. 2). The crossed-lamellar and homo- geneous portions of the shell cannot be designated as separate layers for they vary in extent both between and within a species. In two layered Veneracea the outer part of the outer layer consists of crossed- lamellar structure (Plate 8, figs 2 & 4) which passes transitionally inwards into homogeneous structure (Plate 8, fig. 3). The orientation of the lamels in the outer layer is controlled by the type of shell margin present in each species. In the Veneracea the marginal areas are variable with margins which are reflected, inflected, shelf-like or combinations of these. A further complication to the shape of the shape of the margin may be ribbing and strong concentric sculpture. With a reflected shell margin the first order lamels in the outer region of the outer shell layer lie. Fig. 18. Diagram showing radial sections of three types of shell margin found in the Veneracea. A. Here the margin is slightly reflected; there will be a gradual change from crossed-lamellar structure on the outside to homogeneous structure inwards. B. In this case the margin is strongly reflected and in the position marked by the dotted line there will be a sharp structural change. C. The margin is even more strongly reflected than in B and similarly there will be a sharp change in structure along the dotted
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Keywords: ., 1973, boo, bookauthorbritishmuseumnaturalhistory, bookcentury1900
