Their shells lack a nacreous layer, and the gills are lamellibranch in form.

Sometimes the aragonite forms an inner, nacreous layer, as is the case in the order Pterioida.

Conversely, the nacreous layer is derived from the main surface of the mantle.

The nacreous layer of shells is a complex structure, but rather than being difficult to evolve, it has in fact arisen many times convergently.

The nacreous layer of monoplacophoran shells appears to have undergone some modification.

Lustrin A is an insoluble protein used in the production of a nacreous layer in bivalve molluscs.

It contributes to the properties of the nacreous layer, imparting resistance to cracking and elasticity.

They are covered with a smooth, safe, nacreous layer to let them slip, pearllike, from our souls without real pain.

Furthermore, the nacreous layer also contains some strands of weaker material called growth lines that can deflect cracks.

The shell is relatively soft, and lacks a nacreous layer.