Perfect lateral symmetry is relatively rare in foraminifera though examples can be found in most assemblages. They provide for less exterior surface area relative to chamber volume compared to linear varieties yet do not maximize this relationship as do trochospiral geometries do. Symmetry does on the other hand provide for more even weight distribution and stability in currents within the water. The first example on the left displays the involute biumbilicate, Melonis barleeanus1. Note the presence of septal bridges being the dominant surficial feature rather than sutures due to new growth of chambers over old ones. Sutures can be faintly seen on the surface of the evolute discoidal Spirillina vivipara2, pictured here on the left.
1. (Lipps, J.H. Fossil Prokaryotes and Protists)
2.(Loebelich, A.R. and Tappan, H. Foraminifera of the Sahul Shelf and Timor Sea)