SCLERACTINIA (mid-Triassic through Recent)
living corals with stony skeleton are scleractinia. The ancestors
of this subclass was probably the rugose corals. Like rugose corals
scleractinia may found by themselves as an organism or in a colonial mass.
The colonial form of this sub-class form massive, encrusting, branching
or sheet like structures. They are recognized by their light porous
skelton define by their radialy arrange septa that with sheathing forms
a cup. The development of scleractinia is a distinguishing factor
of this sub-class. A pattern of six septa form then six more form
in between thos folloed by twelve in between the two generations of six
and so on. It wasn't until the Cretaceous that this sub-class formed
in North America. In the Pleistocene the scleracttinia formed in
the area they are found taday; south of the 35th latitude.