Suborder
Calymenina is considered to be the earliest and most primitive
of the Phacopida, primarily because it has some of
the phenotype
chracteristics in common with the
Ptychopariida. Suborders
Calymenina Cheirurina have rostral plates in contrast to suborder
Phacopina where the librigena is a single connected piece.
Schizochroal
eyes are unique to Suborder Phacopina. While they had fewer
lenses (< 700 or so) than the ubuiquitous
holochroal eyes,
the lenses were larger and each one had its own cornea that
extending into a rather large sclera (Treatise, 1997). Schizochroal
eyes appear with the Phacopina in the early Ordovician,
ostensibly evolved from an ancestor with holochroal eyes (Clarkson,
1975).