Systematic position and anatomy of Drepanostomella tucma Hylton Scott, 1948 (Stylommatophora: Scolodontidae)

We redescribe Drepanostomella tucma Hylton Scott, 1948 and discuss the possible affinities of the genus. This is the first published report on the anatomy of a species of the genus Drepanostomella Bourguignat, 1889 since this taxon was previously known only on the basis of its shell morphology and radula. Drepanostomella was traditionally classified within Systrophiidae (now Scolodontidae), a widely distributed family in the Neotropical region. D. tucma is characterized by its deep sunken spire in a nautiloid shell, smooth protoconch, nacreous periostracum and an acute incision at parietal angle of the aperture. The animal has a holopod foot, while most of the other genera components of the family have an aulacopod foot. The radula has a central small tooth with the first lateral tooth smaller than the second one. Noteworthy is the presence of papilla regularly arranged with slightly recurved corneous hooks over the inner penis wall. Similar protuberances with corneous hooks have been already described in a Hirtudiscus Hylton Scott, 1948. Drepanostomella shows shell similarities mainly with Hirtudiscus and Guestieria Crosse, 1872. On the base of its radula morphology, its condition of "egg retainer" and the course of the right ommatophoral retractor passing between penis and vagina, Drepanostomella should be classified within the Scolodontinae (sensu Tillier, 1980). Differences and similarities of Drepanostomella with Happia Bourguignat, 1889, Systrophia Pfeiffer, 1855, Entodina Ancey, 1887 and Systrophiella, Baker, 1925, the rest of the Scolodontinae genera (sensu Tillier, 1980), are discussed. However, more anatomical studies and a cladistic analysis are necessary to further test these proposed affinities.