Comments on the intermandibular muscles of snakes

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Abstract
Some new observations and interpretations are made concerning certain intermandibular muscles in snakes. A muscle occurring in Cylindrophis and Loxocemus, previously identified as part of the M. intermandibularis anterior (herein abbreviated to ima), is actually the M. transversus branchialis (sensu Cowan and Hick, 1951). A deeper muscle found in these taxa is here considered to represent the ima; this muscle is also present in Anilius, Xenopeltis, and uropeltids (Melanophidium, Platyplectrurus), where the ima had been reported as absent. The ima has a similar course, but is typically larger, in pythons. In Calabaria, boas, acrochordids, and Caenophidia, the ima is typically divided into two portions, the pars anterior and pars posterior. It is proposed that this condition is a synapomorphy of these four groups. In acrochordids and Caenophidia (except Dasypeltis), the pars anterior is fully developed and inserts more anteriorly, onto an intermandibular ligament. It is proposed that this condition is a synapomorphy of these two groups. In acrochordids the ligament is deeply bifurcate posteriorly. The M. protractor laryngeus is absent from all Henophidia examined, but present in all Caenophidia. It is suggested that insufficient evidence is available to accept the proposal that a M. geniomyoideus, homologous to that found in anguimorph lizards, is present in snakes. A geniomyoideus-like muscle is present in the non-anguimorph lizard Dibamus novaeguinea.