Astraea (Astralium) kesteveni Iredale is not the same as Astraea (Astralium)
stellare (Gmelin).  A. kesteveni is actually a synonym for A. squamiferum
(Koch).  It is a smaller shell than A. stellare, more depressed, with less
pronounced sculpture.  The operculum is white, not bright blue as in A.
stellare.
Cantharus assimilis (Reeve) is a small species from west Africa.  It is also
found in the Canary Islands.  It is usually dark-colored, with a single row of
white nodules around the shoulder, and reaches about 15 mm.
I never saw a land snail with spines approaching those of Guildfordia; however
there is a genus of land snails, Priotrochatella, which are commonly known as
star shells.  They are depressed in shape like Guildfordia, and they do have
spines around the periphery, but the spines are much more numerous and much
shorter than those of Guildfordia.  It's actually more of a "saw-toothed"
effect than real spines.  These shells are about 1 to 1.5 cm in diameter.
Regards,
Paul Monfils
Rhode Island, U.S.A.