Monetaria annulus from Pemba Is., Tanzania

The freak forms shown here were unknown until about 1998 when few specimens appeared in Tanzania. In the years after, hundreds of shells followed.
The phenomenon has even been given the name "appendiculata".

There are three types of such freaks:

The Hookers: one end of the anterior is prolonged to a hook that normally curls left, rarely right

The Bunnies: Both sides of the anterior form spines or knobs, making the shell look like a bunny.

The Boats: both extremities rostrated, similar to New Caledonian shells

All these freaks are very rare in an extreme state as shown here. The exact origin is so far unknown. Personally, I speculate that the cause of these shells is parasitism.
As the material the callus-accumulations consist of resemble pearls in their elementary components
(see Lorenz 1999: Beitraege zur Kenntnis der Ringkauri Erosaria annulus. Schriften zur Malakozoologie 13: 1-105 pl. 1-6).
A parasite might live in the mantle cavity and cause a signal that there is a disturbance. The repair cells react with accumulating repair-substance, finally forming these unusual appendiculations.