. Bulletin of the British Museum (Natural History). Zoology . n6 J. D. TAYLOR, W. J. KENNEDY & A. HALL. Figs. 76-77. Trigonacea : Neotrigonia margaritacea interior of the shell (76) and radial section (77) showing the general distribution of the shell layers. with a sharp shoulder joining it to the rest of the prism, with which it is in continuity. This boss may show concentric striations. The outer ends of each prism are separated from each other by a wedge of periostracum (Plate 13, figs. 1-2). The boss represents the initial spherulite from which each prism has developed, o
. Bulletin of the British Museum (Natural History). Zoology . n6 J. D. TAYLOR, W. J. KENNEDY & A. HALL. Figs. 76-77. Trigonacea : Neotrigonia margaritacea interior of the shell (76) and radial section (77) showing the general distribution of the shell layers. with a sharp shoulder joining it to the rest of the prism, with which it is in continuity. This boss may show concentric striations. The outer ends of each prism are separated from each other by a wedge of periostracum (Plate 13, figs. 1-2). The boss represents the initial spherulite from which each prism has developed, on the inner surface of the periostracum. In the ribs and tubercles, however, the prisms lose their regularity, and become very much smaller, so that obvious prismatic structure disappears, and the layer is formed in these regions of irregularly im- bricating layers of small granules. In section this structure appears dark grey, with a finely granular appearance. Between the outer prismatic layer and the pallial line there is a middle, aragonitic layer of lenticular nacre. The middle nacreous layer is separated from an inner layer of sheet nacre by a structural discontinuity marking the trace of the pallial myostracum. Prisms are only locally present but there is a definite unconformity of growth layers on either side of the trace of the pallial line. Detailed electron microscope studies were carried out on Neotrigonia dubia and N. pectinata. The mode of preservation of fossil trigoniids from the Jurassic onwards suggests that these were originally aragonitic, as does the preservation of Triassic species of Myophoria that we have 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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