These 2 crusts show 3 structural layers: compact upper layer, porous middle layer and compact lower layer.

这2个结壳从结构上分为3层：致密型上层、疏松型中层和致密型下层。

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These 2 crusts show 3 structural layers: compact upper layer, porous middle layer and compact lower layer.

这2个结壳从结构上分为3层：致密型上层、疏松型中层和致密型下层。

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