How do you spell Thecae

Available Definitions:
1)  n. - A sheath; a case; as, the theca, or cell, of an anther; the theca, or spore case, of a fungus; the theca of the spinal cord.
2)  n. - The chitinous cup which protects the hydranths of certain hydroids.
3)  n. - The more or less cuplike calicle of a coral.
4)  n. - The wall forming a calicle of a coral.