Hi Sue,
 
If your clams have a smooth, shiny periostracum (like Macrocallista maculata),
and are at least 50 mm or so in length, and you are sure they are venerids
(family Veneridae), then they almost have to be juvenile Megapitaria squalida.
There are few smooth venerids in west Mexico.  Other than Megapitaria, the
only ones I can think of are a couple of Pitar species (which are small, 25-35
mm, and more inflated than Macrocallista) and a couple of Tivela species
(which are not shaped at all like Macrocallista - more inflated and more
triangular in outline).  Also, none of these have mottled patterns similar to
Macrocallista maculata.  All the other common genera in that area (Chione,
Protothaca, Ventricolaria, etc.) are sculptured with concentric ridges, as are
most of the local Pitar species.  Megapitaria squalida is usually uniformly
colored, but there is a mottled form that is seen sometimes.  Juveniles of
that color form would look pretty similar to Macrocallista maculata.  I say
juveniles because Megapitaria squalida, when full grown, is a much larger
shell than Macrocallista maculata - in fact, it is one of the largest west
Central American bivalves.  Keen says a large one measures 120 mm, but I have
seen a few over 150 mm.
 
Regards,
Paul M.