Whereas mytiloid prisms are monocrystalline (Griesshaber et al., 2013) and those of ostreoids have a foliated ultrastructure (fig.
In a series of studies we used a battery of microstructural and crystallographic techniques to characterize the pteriomorph calcite prisms where they occur in three extant clades (pterioids, ostreoids+pectinoids, and some mytiloids).
The occurrence of calcite in the shells of pteriomorphs is widespread, although it is notably missing from the arcoids and many mytiloids (Bieler et al., 2014).
Calcitic microstructures have formed continuous layers only in epifaunal taxa (Carter, 1980), and within these, in only four higher taxa, namely the Eupteriomorphs (sensu Boss, 1982), the Hippuritoidea (the extinct rudists), and some mytiloids
and chamoids (Carter, 1980; Harper, 1998).