This variety is distinguished by having elongate valves that are evenly narrowed from the center to the ends of the valves (Stoermer and Yang 1969). The ends of the valves are usually capitate. Other characters are similar to those of the nominate (Stoermer and Yang 1969).
The distributions in Lake Michigan were recorded as the highest between 1937-47 near Chicago and have declined to the present day (Stoermer and Yang 1970). This taxon was uncommon in samples collected in the 1960s. Stoermer and Yang (1970) state that not enough is known about this taxon's ecology to explain its distributions.
This taxa is more common in offshore collections and has the same distribution patterns as the nominate variety (Stoermer and Yang 1969).