The word "lithodomi" is spelled with an "i" after the "th" because it comes from the Greek word "λιθοδόμοι" (lithodómoi). In IPA phonetic transcription, the pronunciation is /liːθodɒmi/. The "th" in this word is pronounced as a voiceless dental fricative (θ), which is the same sound as the "th" in "thin". The "o" in "lithodomi" is pronounced /ɒ/, which is the same sound as the "o" in "hot".
Certain mollusca which bore into rocks and stones, and form for themselves permanent lodgments.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.