. Bulletin of the British Museum (Natural History). SHELL STRUCTURE AND MINERALOGY OF BIVALVIA 265 MACTRACEA (Plate 4, figs I & 2; text-fig. lo) Thirteen species have been examined mineralogically and ten structurally. The shell is entirely aragonite. There are two shell layers in all species examined, an outer crossed-lamellar layer and an inner complex crossed-lamellar layer bounded by the pallial myostracum. In the outer layer, the first order lamels are arranged concentrically and in this super- family are characteristically very fine (Plate 4, fig. 2). The layer is usually very thin a
. Bulletin of the British Museum (Natural History). SHELL STRUCTURE AND MINERALOGY OF BIVALVIA 265 MACTRACEA (Plate 4, figs I & 2; text-fig. lo) Thirteen species have been examined mineralogically and ten structurally. The shell is entirely aragonite. There are two shell layers in all species examined, an outer crossed-lamellar layer and an inner complex crossed-lamellar layer bounded by the pallial myostracum. In the outer layer, the first order lamels are arranged concentrically and in this super- family are characteristically very fine (Plate 4, fig. 2). The layer is usually very thin and worn in the umbonal region but forms most of the hinge. In Spisida solida the crossed-lamellae are finer than in most other species and appear homogeneous in the inner parts of the layer. The inner layer of this species also has a very fine structure and sheets of myostracal prisms are present. These sheets also occur in Raeta undulata. The separation of the two layers is sharp (Plate 4, fig. i) but the pallial myostracum is thin and indistinct in most species. The adductor myostraca are also poorly defined. SOLENACEA (Plate 4, fig. 3) Seven species have been examined mineralogically and four structurally. The shell is aragonitic throughout. Two main shell layers are present, an outer crossed- lamellar layer which forms the hinge and an inner homogeneous layer bounded by the trace of the pallial line. In the outer crossed-lamellar layer the first order lamels are very fine and arranged concentrically to the shell margin over most of the shell. Locally this layer may appear homogeneous. A very thin prismatic pallial myo- stracum, best developed below the umbo is present in all species. The inner layer of all the examples appears homogeneous with a striking lamellate appearance. Electron-microscopy of the inner layer of Ensis siliqua shows that the apparent homogeneous layer is in fact built up from layers of very fine complex crossed- lamellar structure (Plate 4, fig. 3) whic
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Keywords: ., 1973, boo, bookauthorbritishmuseumnaturalhistory, bookcentury1900
