SCLERACTINIA (mid-Triassic through Recent)

    All 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.