Identification Help Please

[Alschapiro]

Hi,

My niece and I found these shells on the beach in SE Florida.  My family has been shelling in the area for 50 years and we have never before found anything quite like them.  I tried to find them in my Audubon guide, but wasn't satisfied with any of my options.  Can anyone tell me what these are?

Thank you,
Amy

[attachment=140][attachment=141] The texture is particularly remarkable
[attachment=142][attachment=143]