>For instance: Rib (costa) is defined as "round-topped elevation......disposed
>colabrally on shell surface".
>But  term rib or costa is often used (also in Treatise) for
>radial linear elevations which are not necessarily round-topped. That
>contradicts the definition given in Treatise.
>It seems that morphological terms are used quite liberally. And also, 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.

Yes, the name tends to vary according to scale relative to a given
specimen (I have gastropod shells which, if placed on top of a lira
on a big shell, would tumble off into the interspace). A very fine
raised spiral line is a lira, riblet or fine cord. A relatively thick
one is a cord. A keel is a cord which protrudes prominently at the
periphery; its crest should be sharpish (note that it need not be a
cord, as a smooth shell with a sharply projecting periphery is also
keeled). Cords can be low or high, rounded, flattopped or
sharp-crested.

Terminology of "radial" sculpture is often abused (as is so common...
think about what a "shock absorber" actually is). Most such sculpture
is COLLABRAL, that is, it follows the growthlines, and as these are
often sinuous, they are not truly axial; therefore collabral rib is
better. Again, such ribs can have any kind of crest, and like spiral
cords, can also carry spines, scales, nodes, gemmae etc.

And as to cephalopod terminology... it should echo gastropod
terminology, but often does not (look up venter, radial, suture).

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