> For monomyarian bivalves (those with a single, coalesced, adductor muscle
> scar), like Spondylus, Pecten, or Ostrea.
>
> 1.      Hold beaks pointing up (dorsal side up, ventral side down)
>
> 2.      Visually draw a mid-line from the beaks to the opposite side
> (ventral).
>
> 3.      The adductor muscle scar is mostly posterior of the mid-line.
>

The major exception to this is Nucinellidae, monomyarian bivalves very
unlike Spondylus, Pecten, or Ostrea.  They are the only monomyarian
bivalves where the remaining adductor seems to be the anterior, and it
is anterior of the midline.  Similarly, if one adductor is
significantly larger than the other, it is usually the posterior that
is bigger, but the solemyoids-close relatives of nucinellids-often
have a larger anterior scar.

--
Dr. David Campbell
Collections Assistant
The Paleontological Research Institution
1259 Trumansburg Road
Ithaca NY 14850

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