This shell measures 5-5/8 in. by 3-1/8 in. Color is mostly black with a white frosted appearance.  Origin unknown.
Picture: http://www.badongo.com/pic/938939
Thanks

This appears to be Busycon carica, the Knobbed Whelk, a common species from the eastern United States. However, the black color is not typical. Shells that have been buried in organic mud for a long time often get stained black like this. That could be the explanation.

http://www.jaxshells.org/busycon2.htm