Many invertebrate skeletons and skeletal fragments in modern carbonate sediments are aragonitic, but, when found in carbonate rocks, they are nearly always calcitic. Two processes of conversion of this aragonite to calcite have been suggested: 1. a process of in situ conversion, and 2. a solution-deposition process in which the aragonite is dissolved and replaced by drusy calcite. This basic process can be further subdivided into four types depending on the relative timing of this solution of aragonitic skeletons and skeletal fragments, lithification of carbonate mud matrix, and deposition of the drusy calcite. If solution of the aragonitic allochems occurs before lithification of the carbonate mud matrix, the resulting void will collapse. Solution-deposition may occur after lithification of the carbonate mud matrix but before any drusy calcite has formed in the originally existing voids within fossils, resulting in fossil replacement without preservation of the outlines of internal cavities. If drusy calcite has started to form in original voids within fossils before the solution conversion process begins, replacements retaining outlines of internal cavities will result. If solution of the aragonite occurs before lithification of the carbonate mud and after drusy calcite infilling of original voids, only internal molds are formed. Originally aragonitic fossils from well cores from the lower Cretaceous of Texas give evidence of all but one of these processes. Important variables in determining which conversion process occurs are original shell structure, fluid chemistry, grain size, and time. The process of conversion has an important bearing on the development of carbonate petroleum reservoirs.