Compare

Semiorbis hemicyclus (if it is, indeed, a distinct taxon) has valves that are slightly less curved and less protracted. Semiorbis catillifera valves are weakly arcuate.

LM scalebar = 10 µm
Semiorbis rotundus LM7 Semiorbis rotundus LM6 Semiorbis rotundus LM4 Semiorbis rotundus LM3 Semiorbis rotundus LM2 Semiorbis rotundus LM5 Semiorbis rotundus LM1
Length Range
32.9-45.1 µm
Width Range
3.9-4.8 µm
Striae in 10 µm
12-13

Semiorbis catillifera

(Morrow) G.Reid and D.M.Williams 2010

Semiorbis catillifera LM1 Semiorbis catillifera LM2 Semiorbis catillifera LM3 Semiorbis catillifera LM4 Semiorbis catillifera LM5 Semiorbis catillifera LM7 Semiorbis catillifera LM6
Length Range
17-40 µm
Width Range
4.8-5.3 µm
Striae in 10 µm
12-13

Also compare to

Semiorbis hemicyclus

(Ehrenb.) R.M.Patrick