Semiorbis rotundus

Valves are strongly curved and semi-circular. Apices are reduced in breadth (3.0-3.8 µm) and subrostrate. Valves at the small end of the size range have a more pronounced dorsal shoulder. Apices are slightly recurved in some specimens. Striae are nearly perpendicular to the dorsal margin, numbering 12-13 in 10 µm. The striae are separated by externally thickened, transapical costae. The costae terminate on the dorsal margin as prominent, planar wing- or spine-like projections. A reduced raphe is present on the ventral valve mantle interface at the apices, but it may not be discernable in LM. Areolae are not visible in LM. Rimoportulae are absent.

There are a number of reports of Semiorbis hemicyclus in North America (Patrick and Reimer 1966, Camburn and Charles 2000, Siver et al. 2005). However, Reid and Williams (2010) examined specimens of Semiorbis from North America and Europe, concluding that S. hemicyclus occurs only in Europe and Greenland, but not in North America. In that work, Reid and Williams described S. rotundus, and determined that this taxon occurs in North America and Japan. We concur that some North American records conform to S. rotundus (Camburn and Charles 2000, p. 21, Pl. 15, Figs. 22-24, Siver et al. 2005, p. 184, Pl. 27, Fig.6 , Pl. 77, Figs. 1-2). We did not, however, examine additional voucher specimens of North American S. hemicyclus. Further work is needed to determine the morphological differences and the geographic distribution of S. hemicyclus and S. rotundus.