The study of palaeoniscoids from Lower Triassic layer of Spitsbergen (Boreosomus reuterskioldi, Acrorhabdus sp., Saurichthys s p.) and from Liassic beds of France (Ptycholepis bollensis) led me to discover in their buccal region a paired ossicle, the labial bone or labial, unknow to date among actinopterygians and other osteichthyan fishes. A detailed analysis of that element and of its relationships with the jaws adductor fossae reveals two important anatomical features : 1) it articulates laterally on a dermal bone (generally called "ectopterygoid", but to be interpreted as the pterygoid), the existence of which had been evidenced only on mesial side of the palatoquadratum ; 2) the lower jaw displays a coronoid process formed by a surangular. Enclosed in the palatomaxillary fossa, which contained the adductor muscle of the lower jaw, the labial bone acted as a block for the reflected fibres of that muscle. Being mobile, it transmitted muscular strenght to the lower jaw with a good efficiency by diminishing the friction. This ossification is probably sinewy in origin, and thus can be compared to the sesamoid bones of mammals. Another element, the parahyoid, already known as a sinewy ossification, is described for the first time in the paleoniscoids. The labial bone is certainly homologous of the non functional labial cartilage of present-day Cladistia (Brachiopterygii). Likewise it has degenerated in other present-day actinopterygians : 1) it is vestigial in polyodontidae ; 2) it possibly incorporated in the coronoid apophyse of the lower jaw in "holosteans" ; 3) it has disappered in teleosteans.