|Basic info||Taxonomic history||Classification||Relationships|
|Morphology||Ecology and taphonomy||External Literature Search||Age range and collections|
If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.
|J. M. Malinky and C.B. Skovsted 2004||Generally cap−shaped shell of near circular to
subtriangular outline, with rounded knob−like apex displaced
to about one third the distance toward the proximal side of the
shell. Angle of dip between apex and both distal and proximal
areas equal in some individuals, but other individuals with
shell curving upward between apex and proximal edge result−
ing in a saddle−shaped profile. Surface with fine, widely and
generally equally spaced concentric ribs and narrow depres−
sions between ribs. Two rod−like and widely diverging ridges
on the internal surface emanating from proximal edge of shell.
Ridges are attached to one side of shell but free on the opposite
side where they terminate in a straight, rounded edge. Termi−
nation of ridges rounded, usually divided into lobes. The shell
is relatively thick (up to 0.35 mm) and the interior surface
often has a finely pitted ornamentation.