03 457-466 Checa

Deposits composed of aragonite prisms, which were formed after the outer shell layer, have been found at the posterior steep slopes of divaricate ribs in two species of Strigilla and another two of Solecurtus. These prisms have their axes oriented perpendicular to the outer shell surface and differ in morphology from fibres of the surface-parallel composite prisms forming the outer shell. They display crystalline features indicating that, unlike crystals forming the outer shell surface, their growth front was free, unconstrained by the mantle or periostracum. These particular deposits are called free-growing prisms (FGPs). In these genera the periostracum is clearly not the substrate for biomineralization and, upon formation, does not adhere to the steep slope of ribs, but detaches at the rib peak and reattaches towards the posterior, just beyond the foot of the posterior scarps of ribs. In this way, a sinus or open space developed between the internal surface of the periostracum and the outer shell surface along each steep rib slope. These spaces could remain filled with extrapallial fluid after the mantle advances beyond that point during shell secretion. FGPs grow within this microenvironment, out of contact with the mantle. Other species with divaricate ribs do not develop FGPs simply because the periostracum adheres tightly to both rib slopes (which are never so steep as in Solecurtus and Strigilla). FGPs constitute one of the rare cases of remote biomineralization in which aragonite is produced and direct contact with the mantle never takes place.