It is reported that in certain features the form of the vomer is significantly different in Caenophidia than in Henophidia (except acrochordids). In Henophidia the vomer typically has one or a few apertures for the exit of the vomeronasal nerve from the bony surround of the vomeronasal organ, well- or moderately-developed vertical and horizontal (palatal) posterior laminae, and only a partially-developed cup-like enclosure for the vomeronasal organ. In Caenophidia the vomer typically has very many tiny foramina for the passage of the vomeronasal nerve, the horizontal posterior lamina in particular is much reduced or absent, and the vomer forms a globular enclosure for the vomeronasal organ. A comparison with the vomer in lizards suggests that the henophidian type of vomer is primitive within snakes and the caenophidian type is derived. Scolecophidia are not discussed. The vomer in acrochordids closely resembles that of Caenophidia, and this form of vomerine morphology is proposed as a synapomorphy indicating the strict monophyly of the group acrochordids-Caenophidia. The acrochordids have been treated very differently by various snake taxonomists and their phyletic position has always been highly problematical. The synapomorphy proposed herein contributes to a solution of this problem.