. Bulletin of the British Museum (Natural History). Zoology . no J. D. TAYLOR, W. J. KENNEDY & A. HALL. nacreous layer Figs. 69-72. Unionacea : Figs. 69-71. Radial sections of Unio mytiloides (69), Quadrula metanevra (70) and Unio pictorum (71) showing the general distribution of the shell layers. Fig. 72. Unio pictorum, general view of the shell interior. that we have examined, a prismatic layer is not developed. This appears to be a primary feature of the genus. In all species where good sections of the prismatic layer were available, the prisms show a typically feathery int
. Bulletin of the British Museum (Natural History). Zoology . no J. D. TAYLOR, W. J. KENNEDY & A. HALL. nacreous layer Figs. 69-72. Unionacea : Figs. 69-71. Radial sections of Unio mytiloides (69), Quadrula metanevra (70) and Unio pictorum (71) showing the general distribution of the shell layers. Fig. 72. Unio pictorum, general view of the shell interior. that we have examined, a prismatic layer is not developed. This appears to be a primary feature of the genus. In all species where good sections of the prismatic layer were available, the prisms show a typically feathery internal structure (text- fig- 75) °f downwardly diverging elements ; this is the " complex prismatic structure " of B0ggild (1930). The prisms also show prominent growth lines. The prismatic layer sometimes interdigitates with the underlying nacreous layer. Resolution of the detailed structure of nacre is largely beyond the limits of optical microscopy, although the two main types can be readily distinguished. " Sheet " nacre occurs in all the species we have examined, usually forming the innermost shell layer, while lenticular nacre forms the middle shell layer of Anodonta anatina, A. cygnea, Potamides littoralis, Quadrula metanevra, Unio mytiloides, and Unio 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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