A dental mouth gag, more commonly used in France, although not invented there and sometimes referred to as Mr Maunder’s gag.
The gag has been cleaned, but the ivory inlay in the handle and the fissuring and appearance of the knots give an indication of the age of the wood.
From ‘Dental Pathology and Surgery’ by James & Salter 1874:
Another arrangement, however, invented by Mr Maunder, seems to possess especial advantages. It is a conical wedge, upon which is cut a spiral screw-worm, gradually increasing in diameter and thickness of the worm. … Where contraction of the mouth is the result of an impacted wisdom tooth, occurring as it does in early life, the teeth in the front of the mouth are almost always firm and will bear the use of of this instrument. Upon introducing the point of Mr Maunder’s gags between the incisors or canines or praemolars, and slowly turning the instrument, it evenly and regularly progresses, separates the jaw, and, as I have though, with quicker results that any other method.