tesselae vs tessellae

Is the Latin word for "mosaics" "tesselae" or "tessellae"? I read it here:

"Tesselae januam ornant."
"Mosaics deck the door."

Also, does this word have a singular? If so, what does it mean? I don't think it's ever used in the singular, just like "trīcæ", which means "toys".