A sarcophagus is a stone coffin. The plural form in Latin is sarcophagi, and in English it is sarcophaguses. Both are correct, though sarcophagi is by far the more accepted spelling.
History
The original Greek word sarkophagos meant “eating flesh”, and was usually paired with the word lithos or stone. They thought a certain kind of limestone of the coffin was responsible for dissolving the corpse inside. Soon sarkophagos by itself was used for any kind of coffin.
Examples
Carved sarcophaguses in marble, elaborate frescoes and a charming wrought-iron latticework gate catch my eye. [The Hindu]
Living quarters are made up of wooden and corrugated iron shacks perched on top of stacked tombs and makeshift beds are made out of stone and marble sarcophagi. [News.Com.Au]
Boxy and frontal, like upright sarcophagi, they were carved, assembled, accessorized, overlaid with deft figurative drawing and bright, patterned paint. [Boston Globe]