Both genera in Bellerophinidae share small, thin, rounded, and bilaterally symmetrical aragonitic shells.[4] Bellerophina is morphologically similar to and likely closely related to the Atlantids, with their shared adaptations and miniscule maximum shell size of 6-8mm making it probable that both genera in Bellorophinidae would have shared their lifestyle as active holoplanktonic predatory micromolluscs.[3][4]
The genus Freboldia contains two species; F. carinii, and F. fluitans.[2][5] It was named after the late Hans Frebold, a German paleontologist who was instrumental in advancing paleontology in the Arctic.[4] F. fluitans dates to the Early Jurassic (Late Pliensbachian/Early Toarcian) of Ellesmere Island.[4] The shells of F. fluitans are small, reaching at most 8 mm in diameter and 6.3 mm in height, and thin, ranging from 25-40 µm in thickness.[4] The shape is described as "inflated, globular, planispiral, bilaterally symmetrical, with deep umbilici."[4] The aperture is kidney-shaped, being taller than it is wide when viewed from the side, with the inner lip curving inwards whilst the outer lip curves outwards.[4] Mature shells are smooth except for straight growth-lines that run parallel to the shell axis at the edges and touch the umbilicus at a single point, whilst young shells have vague spirals and tubercles.[4] A presumed larval shell under 1 mm in size has very thin indents along the whorls lacking in older individuals, which would be unique among known holoplanktonic gastropods.[4] The bilateral symmetry of the shell makes it difficult to tell which direction the whorls coil in.[4] F. carinii is known from the Middle Triassic of the Brembana Valley, very closely resembling the other species except in being much older; if it shared the proposed lifestyle of F. fluitans, it would be the oldest known holoplanktonic gastropod.[5]
The genus Bellerophina has been proposed to contain up to 3 species, including B. minuta, B. recens, and B. vibrayi.[2] However, the classification of B. recens is problematic, and B. vibrayi is a synonym of B. minuta.[2] B. minuta is known from the Early Cretaceous (Albian) of England, originally being incorrectly classified as Ammonites minutus.[2][6] The shell is small, thin, rounded, bilaterally symmetrical, up to 6 mm in diameter, and has a deep umbilicus.[3][4][6] It is ornamented with a grid of raised lines following the curvature, closely resembling larval Oxygyrus inflatus; Freboldia also closely resembles Bellerophina in shape but lacks the lines.[3][4]
