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Fordillidae
Fordillidae is an extinct family of early bivalves and one of two families in the extinct superfamily Fordilloidea. The family is known from fossils of early to middle Cambrian age found in North America, Greenland, Europe, the Middle East, Asia, and Australia.〔〔(The Paleobiology Database Fordillidae entry ) accessed 4 January 2012〕〔(The Paleobiology Database ''Fordilla'' entry ) accessed 4 January 2012〕 The family currently contains two genera, ''Fordilla'' and ''Pojetaia'', each with up to three described species.〔 Due to the size and age of the fossil specimens, Fordillidae species are included as part of the Turkish Small shelly fauna.〔
==Description==
''Fordilla'' are small bivalves with valves that are equal in size and suboval in shape. In size ''Fordilla'' specimens reach a total shell length of up to and a height of .〔 The shells are compressed laterally and the back edge is slightly broadened. The rear adductor is less developed and smaller than the front adductor, while the small pedal retractor muscle scar is positioned near the front adductor scar.〔 The valve hinge is usually straight to slightly convexly curved and each valve will have at most one tooth present. The external surface of the shell occasionally show faint ribbing.〔
Similar to ''Fordilla'', species of ''Pojetaia'' are small, with valves to less than in length. ''Pojetaia'' species have an overall shape which is suboval, with the subequal valves slightly elongated. The ligament is straight with an umbo which is central to subcentral. In contrast to ''Fordilla'' the rear adductor muscle was larger and more developed then the front adductor, with pallial muscles arranged along the valve margins. Also in contrast to ''Fordilla'', valves of ''Pojetaia'' possess between one and three teeth, with up to two teeth per valve. The exteriors of the shells show faint ribbing and fine comarginal growth lines.〔
The inner shell layers of ''Fordilla'' and ''Pojetaia'' species both consist of layers of carbonate, which is akin to the laminar aragonite layer found in extant monoplacophora. The structuring is similar to shell layering found in the extinct genera ''Anabarella'' and ''Watsonella'' which is thought to suggest that members of the phylum Mollusca developed nacre independently several times.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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