1·Somewhat heart-shaped sand-burrowing bivalve mollusks.
稍有心脏状掘沙的双壳类软体动物。
1·There is a case of an oysterlike bivalve that evolved a more and more spiraled shell until, just before extinction, the valves could barely open.
这里有个例子，一种牡蛎状双壳贝，进化出越来越趋螺旋状的外壳，直到该物种灭绝前，其外壳已经几乎打不开了。