SYSTEMATICS OF THE GENUS STREPTACIS (GASTROPODA: HETEROSTROPHA)

The genus Streptacis Meek, 1871 is among the earliest genera in the subclass Heterostropha, one of the most abundant and ubiquitous Recent molluscan subclasses. Streptacis is characterized by a heterostrophic protoconch, a very elongate shape, small size (1 to 14 mm), sinuous growth lines, lack of spiral sculpture, and an anomphalous base. Three species, none of which preserves the protoconch, have been assigned to the genus based on teleoconch morphology. Species can be distinguished primarily based on the orientation of the heterostrophic protoconch, axial sculpture, shape of the aperture, and whorl profile.

The earliest stratigraphic record is one species from the Mississippian of Australia. The type species and two additional species are known from the Pennsylvanian of the USA. Three species are known from the Permian of the USA and three species from the Permian of China. Streptacis sp. has been reported from the Mississippian of Australia, the Triassic of China and questionably from the Pennsylvanian of the USA.

The following species are excluded from the genus Streptacis based on morphological analyses. Microptychis crenimarginis and Microptychis cerithiformis, which were assigned to the genus Streptacis in some of the early literature, have been excluded from the genus based on a wider spiral angle, collabral ribs, and crenulations at the suture. Streptacis gundyensis from the Mississippian of Australia more closely resembles the genus Kimina than Streptacis based on the protoconch, a phaneromphalous base, and a greater width/height ratio. A single external mold of Streptacis ? sp. from the Devonian of the USA was the basis for a Devonian first occurrence of the family Streptacididae in compilations of stratigraphic ranges. However, the specimen conforms more closely to specimens of the murchisonid genus Mesocoelia from the same deposit. Streptacis cf. permiana of Walter, 1953 is excluded from the genus Streptacis based on the presence of spiral lirae, which characterizes the genus Donaldina.