An organic-rich columnar prismatic outer shell layer, which extends far beyond the underlying nacre, has allowed pterioid bivalves (the pearl oysters and their allies) to develop flexible valve margins, allowing a tight hermetic seal when shut. In some taxa, the microstructural arrangement is known to be asymmetrically developed between the two valves. The asymmetry was surveyed across 29 taxa of pterioids (including representatives of known genera) confirming that it is typically the right valve which has a greater expanse of prism-only shell (and less nacre) and showing that this portion of the right valve has more organic content (more than twice the value in some instances) than the equivalent in the left. A more detailed investigation of prismatic material in Pteria penguin comparing the right and left valves revealed that the right valve flange has a higher density of smaller prisms, each with its organic envelope, and not a greater thickness of the organic envelopes themselves. The flange is also thinner on the right valve and shown here to be very flexible when wet. This allows it to bend against the rigid left valve when the shell is closed. Comparison of this structural asymmetry in the pterioids with five outgroup taxa in the Ostreidae and Pinnidae suggests that clades with the asymmetry have been freed from the constraints of a flattened valve morphology and to develop inequivalved forms.

中文翻译：

---

紧密关闭：翼状双壳类动物的灵活瓣膜边缘和微结构不对称

富含有机物的柱状棱柱外壳层远远超出下面的珍珠层，使双壳类蝶类动物（珍珠贝及其盟友）能够形成灵活的瓣膜边缘，从而在关闭时形成紧密的气密密封。在某些分类群中，已知微结构排列在两个阀门之间不对称发展。对翼状体的 29 个分类群（包括已知属的代表）的不对称性进行了调查，证实它通常是右瓣膜，它具有更大的棱柱壳（和更少的珍珠层），并表明右瓣膜的这部分有更多有机含量（在某些情况下是该值的两倍以上）比左侧的等价物高。对 Pteria penguin 中的棱柱材料进行的更详细的研究比较了左右瓣膜，发现右瓣膜法兰具有更高密度的较小棱柱，每个棱柱都有其有机外壳，而不是有机外壳本身的厚度更大。右侧阀门的法兰也更薄，此处显示在潮湿时非常灵活。这允许它在外壳关闭时抵靠刚性左阀弯曲。将蝶类中的这种结构不对称性与 Ostreidae 和 Pinnidae 中的五个外群分类群进行比较，表明具有不对称性的进化枝已经摆脱了扁平瓣膜形态的限制并发展出不等价的形式。右侧阀门的法兰也更薄，此处显示在潮湿时非常灵活。这允许它在外壳关闭时抵靠刚性左阀弯曲。将蝶类中的这种结构不对称性与 Ostreidae 和 Pinnidae 中的五个外群分类群进行比较，表明具有不对称性的进化枝已经摆脱了扁平瓣膜形态的限制，并发展出不等价的形式。右侧阀门上的法兰也更薄，此处显示在潮湿时非常灵活。这允许它在外壳关闭时抵靠刚性左阀弯曲。将蝶类中的这种结构不对称性与 Ostreidae 和 Pinnidae 中的五个外群分类群进行比较，表明具有不对称性的进化枝已经摆脱了扁平瓣膜形态的限制，并发展出不等价的形式。