These curious remains have been lately made the subject of detailed research, and it is now concluded that they form a group probably ancestral both to the sponges and the corals. They are calcareous, and generally cup-shaped or conical, often furnished at the pointed base with roots or strands for attachment to the surrounding reef. They have two walls, both the inner and the outer being perforated like sponges. As in the corals, they are divided by transverse septa and these are also perforated. Certain of the genera as _Protopharetra_ (Fig. 67 D), _Coscinocyathus_, and _Archaeocyathina_, are common to the Cambrian of Sardinia and South Australia, whilst other genera of the class are also found in Siberia, China, Canada and the United States. A species of _Protopharetra_ was recently detected in a pebble derived from the Cambrian limestone in the Antarctic, as far south as 85 deg. An _Archaeocyathina_ limestone has also been found in situ from Shackleton's farthest south. _CORALS_ (Class Anthozoa). =Rugose Corals.--= Many of the older types of Corals from the Palaeozoic rocks belong to the Tetracoralla (septa in multiples of four), or Rugosa (i.e., with wrinkled exterior). =Ordovician Corals.--= In Great Britain and North America Rugose Corals are found as early as Ordovician times, represented by _Streptelasma_, _Petraia_, etc. In Australia they seem to first make their appearance in the Silurian period. =Silurian Corals.--= In rocks of Silurian age in Australia we find genera like _Cyathophyllum_ (with single cups or compound coralla), _Diphyphyllum_, _Tryplasma_ and _Rhizophyllum_, the first-named often being very abundant. The compound corallum of _Cyathophyllum approximans_ presents a very handsome appearance when cut transversely and polished. This coral is found in the Newer Silurian limestone in Victoria; it shows an alliance with _C. mitchelli_ of the Middle Devonian of the Murrumbidgee River, New South Wales (Fig. 69 A). =Silurian Hexacoralla.--=