Tenagodus obtusus is a curiously shaped slit-worm shell with the first few whorls coiled regularly but suddenly shifting to growing an uncoiled tube. It is mainly known from the Mediterranean Sea with specimens from Atlantic waters generally attributed to T. senegalensis (G. B. Sowerby II, 1876), although some authors claim that the two should be considered synonymous. Tenagodus senegalensis appear to have generally more tightly coiled earlier whorls, although they are quite variable indeed. It is an obligatory sponge comm.ensal and spends the entire life inside the host sponge, in most cases Holoxea furtiva Topsent, 1892. More than one specimens is usually found together in a cluster within the sponge. Most of the shell is completely embedded in the sponge which provides shelter and protection, only the uncoiled tube extends out. It is found in intertidal and shallow subtidal waters down to about -50m, and is a filter feeding gastropod. There is a corneous operculum supported by a much reduced foot, which is heavily sculptured with opercular bristles, the form of which is an important taxonomic character in this family. It is very similar in general appearance to the Indo-West Pacific species T. ponderosus Morch, 1861, but is usually readily distinguishable by the slit morphology -- the slit of T. ponderosus is a series of elliptical holes on the coiled whorls then gradually becoming denticulated and then finally smooth-sided at the uncoiled part, whereas that of T. obtusus is smooth-sided without denticulation throughout the entire teleoconch. Typical shell length around 100mm., extremely large specimens may exceed 160mm. Although generally a comm.on species, it is uncomm.on to find one in good condition on the shell trade market.