The external orofacial morphology, perioral bristle distribution and feeding behaviour of Dugong dugon, Trichechus inunguis, T. senegalensis and T. manatus are described. Despite their differing orofacial morphology, dugongs and trichechids possess six similar regions: the oral disk, orofacial ridge, supradisk, chin and upper and lower bristle (modified vibrissae) pads. All living sirenians possess six discrete fields of bristles: four on the upper lip (U1, U2, U3, and U4) and two on the lower lip (L1 and L2). The distribution of these fields is similar among all living sirenians with the exception of the U1 bristle fields in dugongs. Perioral bristle field boundaries are distinguished by distinct changes in their length-to-diameter ratios. Both dugongs and trichechids possess bristle-like hairs covering the oral disk, which possess length-to-diameter ratios intermediate between perioral bristles and postcranial hairs. Sirenians use elaborated facial musculature in conjunction with perioral bristles to acquire, manipulate and ingest aquatic vegetation. The U2 and L1 fields are the primary bristles used to ingest vegetation. The use of the L1 bristle fields is similar among all living sirenians. However, dugongs and trichechids are divergent in their use of the U1 and U2 bristle fields. Dugongs use the U2 bristles fields in a medial-to-lateral motion, while all trichechids use the U2 bristles in a prehensile, lateral-to-medial, grasping motion. These divergent behaviours presumably allow dugongs to exploit benthic foraging (i.e. consumption of rhizomes) to a greater degree than trichechids. Functional hypotheses of rhizome excavation are presented for both dugongs and trichechids.