I agree with Mojo, conch or least some species of Strombus. That's why I originally said if smooth all the way around. A Conus sp will look identical to a Conch with that one exception. The Conch will have the ridged posterior or along the shell opening and a more defined "swirl".
Other than the probosis, have you seen two eye stalks like that of a hermit?
Cheers
Steve