The Nerineacea form a distinctive and significant component of shallow marine Mesozoic Tethyan macrofaunas. They occur in shelf sediments deposited in tropical regions during the Jurassic and Cretaceous. The group first appears in the early Jurassic and high diversity levels had developed by late Jurassic and early Cretaceous times. A major extinction episode occurred during the latter part of the Cenomanian and the Turonian, with the Nerineacea becoming finally extinct in the Maastrichtian. The heterostrophic nature of the ncrineacean protoconch (described here from Nerineidae and Nerinellidae species) indicates opisthobranch affinities. The Nerineacea is placed in the Entomotacniata, an independent order within the Opisthobranchia. The Entomotaeniata is considered to show greatest affinities with some members of the Acteonacea. The Pyramidcllacea are not included in the Entomotaeniata. The order contains the Ceritellidae, Nerineidae, Nerinellidae and ltieriidae. Early phylogenetic separation of the first three families is indicated. Within each family a limited number of "stable" internal fold patterns developed independently which reflected anatomical features of the abapical portion of the nerineid animal. The Nerineacea typically inhabited warm, clear, carbonate-dominated shallow marine environments on and around off-shore "highs", however, some genera lived in prelittoral or lagoonal situations. Only Ptygmatis shows any evidence of tolerance of abnormal salinities. Carbon and oxygen isotope analyses demonstrate that Nerinella and Simploptyxis specimens from Austrian Gosau deposits lived in water of normal marine salinity. Most Cretaceous nerineid genera were probably mobile epifaunal herbivores, although Nerinellidae species may have been infaunal. Cretaceous Nerineacea are taxonomically reviewed; the diagnoses of the subfamily Umboneinae; Diozoptyxis, Adiozoptyxis; Julesia and Phaneroptyxis are revised. Stratigraphic and palaeogeographic ranges indicate that certain species could potentially be used for correlation. Quantitative measurements of various parameters are included in specific descriptions; the value of these in specific resolution is tested in appropriate cases. An analysis of morphological range in one species (Diozoptyxis cochleaeformis) demonstrates wide continuous variation in overall shell shape and whorl concavity. However, external ornament and internal fold pattern do not show significant intraspecific variation; these features therefore provide reliable characters for species definition.