"there seem to be no sharp boundary between thread, cord, costa(rib),
carina(keel). They mostly differ by their width and prominence which is very
subjective.
What is a “cord” for me can be called “rib” by someone else."

Not only is the above statement true, but such terms can also be relative to
shell size. A linear elevation of a given height and width on a very small
shell might qualify as a "rib", while a linear elevation of the same
dimensions would only be a "cord" on a larger shell, or a "thread" on a very
large shell.

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