Scanning electron and petrographic studies of Thamnophyllum, Pachyphyllum, Tabulophyllum, and Charactophyllum indicate that the Rugosa have much in common with the Scleractinia in terms of formation and structure of their exoskeleton. Possibly all rugosan septa are trabecular in construction, as lamellar structures are secondary, and fibro-normal structures are at least partly the result of modification of trabecular septa by diagenetic processes. There is no difficulty in applying the scleractinian model of biocrystallization to the rugosans. The question of original skeletal mineralogy remains unanswered, with some evidence of occurrence and microarchitecture suggesting that it was calcitic. However, recent study of aragonite inversion or recrystallization to calcite in scleractinians shows that structures produced by these processes are close to those noted in the Rugosa.