Seashell: Tusk Shell
It is quite amazing to find that those spiraling mollusks also developed into a shell that doesn't spiral so much, just giving a slight curve, like a tusk. This type of shells are indeed called Tusks.
This one is called Boar's Tusk (Dentalium Aprinum). This one measures 8 cm, and is bought in Taiwan, 2003. It looks like a jade jewel.