Collection of over a dozen specimens of Hyolithes aclis Hall from the Middle Devonian of New York indicates that this species belongs to a new genus, Hallotheca, to which the species Hyolithes halli (Hall) is also assigned. However, the generic assignments of the Silurian species Hyolithes cliftonensis Foerste and the Devonian species H. centennialis Barrett, H. ceratophilus Clarke, H. neapolis Clarke, and H. triliratus Hall are uncertain because of poor preservation of their type specimens. Use of these names should be confined to the types until better preserved topotypes become available. Finally, H. cooperi (Williams) consists of organisms of uncertain affinities, and the type of H. principalis Hall may be a fragment of Devonian sandstone rather than an organism.