Sphenothallus Hall, 1847 is a widespread Paleozoic marine taxon that has been interpreted most recently as a tubiculous annelid or other ‘worm’ or as a thecate hydrozoan or scyphozoan cnidarian (e.g., Mason and Yochelson, 1985; Feldmann et al., 1986; Van Iten et al., 1992, 1996; Neal and Hannibal, 2000; Zhu et al., 2000). Members of this genus are characterized by a very gently tapered, finely lamellar apatitic tube bearing a closed subconical holdfast and a pair of robust longitudinal thickenings situated at the ends of the tube's greatest diameter (Zhu et al., 2000). Rarely, the tube exhibits an internal transverse wall that extends adaperturally along the inner surface of the tube proper and may also exhibit a subcylindrical terminal protuberance (Van Iten et al., 1992, figs. 5, 6). The transverse wall of Sphenothallus is essentially identical in gross morphology and microstructure to the schott (apical wall) of conulariids, an extinct group of thecate cnidarians (Van Iten, 1991, 1992a, 1992b; Jerre, 1994; Van Iten et al., 1996; Hughes et al., 2000) that may have been closely related to Sphenothallus (Van Iten et al., 1992). Clarke (1913, pl. 26, figs. 16-18; see also Van Iten et al., 1992, fig. 3) illustrated a compound specimen of S. sica consisting of numerous “daughter” tubes arranged in a highly regular manner (in opposition and evenly spaced) along a single, relatively large “parent” tube whose test wall appears to be confluent with the base of the “daughter” tubes. This compound specimen is distinctly different from the more common associations of epibiontic, holdfast-bearing Sphenothallus arranged in a less orderly fashion on Sphenothallus tubes or other shells (see for example Feldmann et al., 1986, fig. 2), and probably is best interpreted as a clonal colony. Sphenothallus tubes, both solitary and branched, are most similar to thecae of solitary and colonial hydrozoan and scyphozoan polyps, differing from these relatively delicate structures mainly in being mineralized and in bearing a pair of longitudinal thickenings. Like conulariids, Sphenothallus occurs in rocks originally deposited in a broad spectrum of marine facies (open carbonate shelf to restricted shale basin), but is perhaps most conspicuous in restricted basin and shelf slope facies represented by dark shales and lime mudstones.