. Bulletin of the British Museum (Natural History). 270 TAYLOR, KENNEDY AND HALL VENERACEA (Plate 7, figs 1-5; Plate 8, figs 1-5; Text-figs 17-22) This superfamily includes a large number of extant genera and species, conse- quently we have examined over fifty species structurally and mineralogically. The shell is aragonitic in all species. Both Boggild (1930) and Oberling (1964) stated that the distribution of shell structure types is highly variable, thus in order to ascertain if there is any sj'stematic variation we have listed and discussed the sj>ecies examined at family and sub-family
. Bulletin of the British Museum (Natural History). 270 TAYLOR, KENNEDY AND HALL VENERACEA (Plate 7, figs 1-5; Plate 8, figs 1-5; Text-figs 17-22) This superfamily includes a large number of extant genera and species, conse- quently we have examined over fifty species structurally and mineralogically. The shell is aragonitic in all species. Both Boggild (1930) and Oberling (1964) stated that the distribution of shell structure types is highly variable, thus in order to ascertain if there is any sj'stematic variation we have listed and discussed the sj>ecies examined at family and sub-family level. (Table 15). The shell structural variations found are indeed more variable than any other superfamily. The various combinations are shown diagrammatically in Text-fig. 17. The apparently most important structural distinction is that between species having an outer composite prismatic layer and those without. The other variations exhibited between crossed-lamellar, complex crossed-lamellar and homogeneous structures almost always show transitions and all gradations between these structures may be found. There is thus in many species a basically three layered shell consisting of an outer composite prismatic layer, a middle crossed-! amellar/homogeneous layer and an inner complex crossed-lamellar/homogeneous layer. In most other species the shell is basically two layered with an outer crossed-Iamellar/homogenous layer and an inner complex crossed-lamellar/homogeneous layer. The composite prismatic layer consists of radially aligned primary units made up of smaller crystallites radiating from a central axis (Plate 7, figs 1-5). Each of these smaller crystallites may be several mm. in length and 40 jx in diameter, but the size varies greatly from species to species. Each of these crystallites is surrounded by a sheath of organic matrix. Closer examination shows that each of the larger crystallites cp- 'h' h Fig. 17. Diagram showing
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