The investigation of the development of the trigeminal jaw adductor musculature in the turtleChelydra serpentinadocuments the early aggregation of muscle rudiments around the innervating nerve branches, probably a consequence of inductive interaction. This may explain the early continuity of the intramandibularis with the intermandibularis muscle. Several aspects of muscle development differ in the turtle as compared to lizards. These differences highlight the fact that conjectures of homology, based on a static topographical correspondence of adult structures, cannot capture the dynamics of the developmental process. The intramandibularis muscle of turtles, comparable to that of crocodiles, represents a plesiomorphous structure which is not homologous to the intramandibularis muscle of lacertoid lizards, a derived feature of the Lacertoidea. A derived feature of the chelonian jaw adductor musculature is the posterodorsal expansion of the external adductor along a supraoccipital crest, developing according to a pattern of Haeckelian recapitulation. Muscle development serves to corroborate the concept of a monophyletic Eureptilia, including diapsids and synapsids, as opposed to the (paraphyletic) Anapsida. The impact of the differentiation of the external adductor into a pulley system on cranial kinesis is analysed in biomechanical terms.