What is the most elevated part of a bivalve shell, or the part in which the hinge is situated. Summit level, the highest level of a canal, a railroad, or the like, in surmounting an ascent called?

summit — (Zoöl.) The most elevated part of a bivalve shell, or the part in which the hinge is situated. Summit level, the highest level of a canal, a railroad, or the like, in surmounting an ascent. (Webster's 1913.)

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