Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.3. Microscopic structure of shell
When viewed in a cross section, the shell presents three distinct layers: periostracum, prismatic layer and nacreous or pearly layer.
Periostracun It is the outermost, greenish-brown, thin, translucent, horny layer, formed by a chitin-like horny organic substance, the conchiolin. It is secreted first by the edge of the mantle. It is rough and serves to protect the underlying calcareous layer, from being dissolved by the corroding action of weak carbonic acid in water. It gives the exterior of the shell most of its colour. It is often eroded or worn away from older parts of the shell, like umbo, where the median prismatic layer becomes exposed.
Prismatic layer. The middle or prismatic layer is also secreted by the mantle-edge. It consists of minute prisms or crystals of calcium carbonate, prismatic separated by thin layers of chiolin, arranged perpendicularly nacre to the surface of the shell It gives strength to the shell.
Nacreous or pearly layer The innermost layer called nacre or "mother-of-pearl', is secreted by the whole outer surface of the mantle and present a smooth, iridescent or lustrous surface.

It consists of alternate of the shell and the mantle, layers of calcium carbonate and conchiolin, laid down parallel to the surface of the shell. The mantle deposits nacreous layers over any irregularities that occur either in the shell or over loose particles that may lodge in the mantle itself. The result is the formation of a pearl.
The proportion of the CaCO3 in the entire shell varies from 89-99, whereas that of phosphate of lime from 1-2%. The chitin like-horny organic base, called conchiolin provides a sort of membranous framework.