Four new species of bathyal acrothoracican barnacles of the family Lithoglyptidae are described: Lithoglyptes viatrix from Hawaii, L. tectoscrobis and Weltneria ligamenta from Tonga, and W. griggi from the Caribbean island of Grenada. All burrow in ahermatypic corals of the genus Enallopsammia. Because rostra are very rare in the Acrothoracica, the rostra and calcareous cement possessed by at least three of these species are of particular interest. Instead of serving simply to attach the barnacle to the burrow wall, they may be modified intoa roof (or trail) sealing the apertural slit left behind as the barnacle bores across the coral surface. This system is so highly developed in L. viatrix that some specimens are truly motile and abandon the older parts of their burrows. Other rostral structures in living and fossil acrothoracicans are reviewed, and criteria are given to distinguish rostra from calcareous cement. Analogous systems of calcareous structures (cement only) and apertural motility in the families Cryptophialidae and Trypetesidae, and in various fossil genera are also discussed; apparently none of these acrothoracicans abandon any part of their burrows, although the bulk of the mantle may move a considerable distance. A general trend towards loss of the rostrum and calcareous cement is evident in extant and fossil Acrothoracica that live in shallow environments, but at different stages of reduction the remaining calcareous parts often have been independently modified to protect the mantle as the aperture moves to a more favorable position in the substrate. Calcareous parts in fossil acrothoracican burrows are not always ichnofossils, but may be true body fossils, a fact which has nomenclatural significance.