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The musculature of the jaw, branchial, and shoulder regions of the ratfish, Hydrolagus colliei, was dissected and described in an effort to determine possible homologous characters shared by the shark, a member of the subclass Elasmobranchii, and the ratfish, a member of the subclass Holocephali. The musculature of the ratfish jaw differs from the morphological pattern found in sharks in that the jaw muscles of the ratfish are located anterior to the orbit, unlike the jaw muscles of sharks, which are posterior to the orbit. It is suggested that this anterior musculature is a character that was shared by fossil ptyctodonts (Class Placodermi) and is not found in any other living fish today. It is further suggested that the labial cartilages of the ratfish may be remnants of the palatoquatrate cartilage which were excluded as the jaw became fused to the braincase. The morphology of the head of the ratfish best supports the hypothesis that ratfish and ptyctodonts are more closely related to each other than either taxon is to sharks or other jawed fishes.



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