I have also puzzled over this davidis/major thing in years past, but
after examining hundreds of specimens from a variety of localities, I
have yet to see a specimen which was sufficiently different from the
typical range of variation of Harpa major to justify calling it by a
different name.  My experience has been identical to that of Mr. Frydman
- some specimens have a single large ventral blotch, some have the
blotch divided by a light band, forming 2 blotches, and others have two
light bands, separating the dark blotch into three.  However, I don't
think this justifies the statement "H. davidis always has three separate
blotches when H. major can have one or two".  If the shells are
otherwise identical (or show a similar range of conchological
characteristics), then it seems to me the more valid statement would be
"this species can have one, two, or three ventral blotches".  Why give
the specimens with three blotches a different name?  If that is valid,
then shouldn't the 1-blotch and 2-blotch forms have separate names as
well?  Color markings, in the absence of other significant morphological
differences, are not valid grounds for taxonomic separation.  So, until
additional evidence comes my way, I consider Harpa davidis at best a
color variation of Harpa major, but more likely just a synonym.

Paul M.