The word "Phalanx" is the phonetic Latin spelling of the Greek term. The correct term is FA/LLAGX, i.e. phalagx, from FA/LAGGOJ, i.e. phalaggos. The double gamma is pronounced "enga," which is rendered "ng" in English.

A phalaggos is a roller. Rollers were heavy weights that were used to smooth out the ground from ruts. The term is most descriptive. A formation 500 men across and 8 deep would be a "roller". Phalanx or phalagx are both correct. The trend right now among classicists is to use phalagx.

Greek scholars, prefer strict transliteration schemes to preserve the phonetic qualities of ancient Greek, for both proper names and Greek terms (it also helps when they translate).

Consequently, write "Thoukydides" instead of Thucydides; Ploutarkhos as opposed to Plutarch; Platon vs. Plato; Akhilleus vs. Achilles; Homeros instead of Homer ... and so on is you want to be a correct classicist. Correctness in this case is a matter of taste.