Identification please New Jersey

[crumpybumpy]

Hi I'm new to shell collecting and I'm just starting out in New Jersey USA.  These 3 are all the same, correct?  Why is the one on the right very smooth, almost an enamel/ivory texture compared to the other 2 which feel more calcium-carbonate/chalky?  And what are they?  Thank you.

[paul monfils]

These are Astarte shells. That is both the common name and the genus name (first part of the scientific name).  These are Astarte castanea, the "smooth astarte" or "chestnut astarte".  This species sometimes lives in fairly shallow water, but more often in deeper water, down to about 100 feet depth.  In life it has a shiny brown fibrous covering, or "periostracum" over the outside surface of the shell.  It is partially worn off the two shells on the left, completely removed from the shell on the right.  This happens when the shells get tossed around with sand by the surf.

Here is a picture of various types of Astarte shells, including yours: http://www.conchology.be/?t=65&family=ASTARTIDAE