Knobbed Whelk

I think this is actually a Knobbed Whelk.
They are somewhat similar in looks but there are a couple of differences.
Lightning Whelks spiral to the left. This one clearly spirals to the right. Also, the knobs are larger on the Knobbed Whelks.

Here are some references:
http://www.okeefes.org/Whelks/Whelks.htm...
http://www.sms.si.edu/irlspec/Busycon_co...

I'm not sure but I believe this is one of the species of Lightning Whelk...http://en.wikipedia.org/wiki/Lightning_whelk

Wonderful !

http://en.wikipedia.org/wiki/Conch

Its a conch and its edible.