Mollusks: (gastropods, pelecypods, cephalopods)
Composed initially of aragonite, allochems of the major mollusk groups occur as open molds (secondary pores) or molds filled with calcite spar (replacements) in most rocks older than the Pleistocene. Rare preservation of aragonite is known, even from Paleozoic rocks, but the majority of these allochems are vastly modified by diagenesis. Allochems from these groups are especially difficult to identify when broken into small bits.

Mollusks: (rudists, oysters)
The extinct rudists and extant oysters are an exception to the general rule that mollusk shells are made of aragonite. These groups secrete calcite fibers that are generally arranged parallel to the shell wall, as in brachiopods. Additional complexity in the rudists however comes in the form of porous layers and also interlaminations of aragonite that, in its usual fashion, can be replaced by sparry calcite. Discrimination from brachiopods is not generally a problem because calcitic mollusks are a feature of Mesozoic, Cenozoic and Quaternary rocks in which brachiopods play a diminished role. These mollusks are also characterized by a greater size and shell thicknesses that generally exceed those of brachiopods.