Horn corals
Scientifically these corals are known as the Rugosa, to fossil hunters they are horn corals because of a unique horn-shaped chamber with a wrinkled, or rugose, wall.
Because of their shape the Rugosa are one of the most well-known and easily recognizable corals.
They are an extinct order of coral that were abundant in Middle Ordovician to Late Permian seas.
Some rugosans reached nearly a meter in length.
Like modern corals, rugose corals lived on the sea floor or in a reef. They had tentacles to help them catch prey.
For more information try these links:
http://www.uky.edu/KGS/fossils/corals.htm
http://members.wolfram.com/jeffb/Fossils/Cnidaria.shtml
http://www.ucmp.berkeley.edu/cnidaria/rugosa.html