. Bulletin of the British Museum (Natural History). Zoology . SHELL STRUCTURE OF THE BIVALVIA in A very thin, discontinuous, prismatic pallial myostracum has been observed in most of the species that we have examined. Sections through the adductor muscle scars of Etheria and Margaritifera margaritifera showed thin prismatic myostracal layers in these areas. A striking feature of Anodonta cygnea, Etheria sp., Etheria elliptica, Hyria ligatus, Margaritifera margaritifera and Unio fisherianus is the development of thin (o-oi- 0-03 mm) periostracum-like, conchiolin layers within the n
. Bulletin of the British Museum (Natural History). Zoology . SHELL STRUCTURE OF THE BIVALVIA in A very thin, discontinuous, prismatic pallial myostracum has been observed in most of the species that we have examined. Sections through the adductor muscle scars of Etheria and Margaritifera margaritifera showed thin prismatic myostracal layers in these areas. A striking feature of Anodonta cygnea, Etheria sp., Etheria elliptica, Hyria ligatus, Margaritifera margaritifera and Unio fisherianus is the development of thin (o-oi- 0-03 mm) periostracum-like, conchiolin layers within the nacre. These conchiolin sheets probably serve to preserve the frequently abraded and etched umbonal area. Figs. 73-74. Fig. 73. Margaritifera margaritifera, radial section showing the general distribution of the shell layers and the conchiolin sheets in the nacre. Fig. 74. Detail of organic ' conchiolin ' sheets and prismatic layers in the hinge area. Radial section. against solution, and perhaps also strengthen the umbonal and hinge area, replacing the now-missing periostracum. In M. margaritifera these periostracum-like sheets are confined to the inner layer of the nacre and are associated with thin (0-12-0-10 mm. prismatic layers (text-figs. 73-74). Each prismatic layer lies immediately inside the adjacent conchiolin layer. It appears that these organic layers have the same structure as the periostracum, for the sequence of periostracum-prisms-nacre is the same as that occurring at the growing edge of the shell. In the umbonal region, these conchiolin bands pass through the internal ligament, although here the under- lying prism band is absent. The actual stimulus for the laying down of these conchiolin layers is not known. Beedham (1965), from regeneration experiments carried out on Anodonta cygnea, has found that the general outer mantle surface which normally secretes only nacreous structure, can, upon injury, repair the shell with layers of periostracum, prisms and nacre
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