Tetractinellida

Tetractinellida, an order of Sponges, including those with a skeleton composed of spicules composed of four rays. The spicules may be free or united by interlocking processes or some intermediate substance; in rare cases the skeleton is absent. The order is also known as the Spiculispongiae. Most of the members of the group are extinct; but a few, such as Tethya, Geodia, and Halisarca, are still living. It dates from the Silurian period onward.