. Bulletin of the British Museum (Natural History). Zoology . 98 J. D. TAYLOR, W. J. KENNEDY & A. HALL crossed-lamellar layer, with concentrically arranged first order lamels, forms the hinge and teeth, while the pallial and adductor myostraca are prismatic aragonite. There is also an innermost, aragonitic layer, which is rather complicated and varied ; a crossed-lamellar structure with concentrically arranged lamels is dominant, but this is interrupted by lenses or patches of myostracal type prisms in two of the species that we have examined, Spondylus gaederopus and 5.
. Bulletin of the British Museum (Natural History). Zoology . 98 J. D. TAYLOR, W. J. KENNEDY & A. HALL crossed-lamellar layer, with concentrically arranged first order lamels, forms the hinge and teeth, while the pallial and adductor myostraca are prismatic aragonite. There is also an innermost, aragonitic layer, which is rather complicated and varied ; a crossed-lamellar structure with concentrically arranged lamels is dominant, but this is interrupted by lenses or patches of myostracal type prisms in two of the species that we have examined, Spondylus gaederopus and 5. nicobaricus. These lenses of prisms break up laterally into prismatic myostracal pillars, and may give rise to them towards the shell surface. In all the species examined, the myostraca may give rise to short, pillar-like extensions ; those arising from the pallial myo- prismatic myostraca outer foliated layer / calcite pallial adductor. middle crossed lamellar inner crossed lamellar layer layer Fig. 60. Radial section of Spondylus calcifer showing the general distribution of the shell layers etc. stracum are directed only inwards, but those arising from the adductor myostracum pass in both directions. Thin, prismatic myostracal bands are developed in the umbonal region of the crossed-lamellar layers, outcropping just below the hinge. These are the trace of the mantle attachment in this region. The first order lamels of the middle and inner shell layers show a striking correspondence and continuity across the pallial myostracum (PI. 4, fig. 1). In all the species examined, tubules are present in all shell layers and throughout the whole shell, although they are most abundant within the area bounded by the pallial line. The calcified portions of the resilium of seven species of Spondylus were examined and found to be aragonitic in all cases. PLIGATULIDAE (Family text-figs. 61-62) Two species were examined, both mineralogically and optically. The shell consists of both calcite an
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