Hello Flavio,
Although the complete absence of spines on the keeled periphery is unusual, I
still suspect that your Astraea is a juvenile Astralium haematragum (Menke).
Small juveniles of this species are usually depressed, the width of the shell
exceeding the height.  Gradually they become "taller" until, in fully mature
specimens, the height exceeds the width.  The young ones often show a keeled
shoulder like your specimen, but there are usually small spines along the
shoulder as well.  I do have a few juvenile specimens where some sections of
the shoulder are spined, and other sections spineless.  The basal sculpture
(scaly spiral cords, heavier cords alternating with lighter ones), the
coloration, and the operculum of your specimen are all typical of A.
haematragum.  Also, in your first image, the axial ribs which characterize the
adult shell can be seen developing in the area above the aperture.  If your
shell is a described species, I believe this combination of characteristics
would rule out anything but A. haematragum.
Regards,
Paul M.