Flexopecten flexuosus versus Fl. glaber
  Conchological details




Lower auricle

The squamous crest is more arched
in Flexuosus than in Glaber.
The wing is flatter in Glaber.
Auricular trench

Deeper in Flexuosus.
If you compare with the pics above,
you will notice that its wall is
thinner in Fl. glaber.
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Right valve

Each rib has two lateral ridges in Glaber
and the furrows of Flexuosus tend to vanish
near the margins while they become
more and more visible in Glaber.
Upper umbonal area

Flexuosus always looks smooth,
Glaber sometimes looks cancellated.
Left valve

is more triangular in shape
than Glaber.
Byssal notch and ctelonium

What could we learn with these ctelonia ?
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  Additional distinctive features
  Flexopecten flexuosus:

1- The auricular trench is deeper than in glaber, and is prominent in the inner side.
2- The margins smooth out the inner relief of the valves (ribs, undulations).
3- The growth lines are never noticeable.
4- The byssal notch is quadrangular and deep.

Flexopecten glaber:

1- Inner furrow sometimes appearing behind the auricular trench, whose wall in thinner than in flexuosus.
2- The inner relief of the valves is more accentuated near the margins than elsewhere.
3- The growth lines can give to the shell a cancellated sculpture near the umbos.
4- The byssal notch is triangular.

Some other features seem less useful, and won't be repeated.
Many thanks to Carlos Duares de Carvalho (website here) and Paolo G. Albano (website here), who gave me pics and shells. Next time we'll try to point out the differences between a Conus (Mollusca, Gastropoda, Conidae) and a Patella (Mollusca, Gastropoda, Patellidae). Tricky: both are conic !