Head almost entirely covered by network of intersecting rugae (a), lacking long, well-organized and parallel longitudinal rugae on the frons (b). Frontal carinae indistinct, quickly becoming integrated into dense rugoreticulum that covers the entire face. Antennal scrobes entirely lacking. Antennal insertions surrounded by deeply excavated pits (c). Diagnostic characters of P. obscurithorax major workers among introduced Pheidole.