~_M. depressor mandibulae._~--The depressor of the lower mandible is the sole muscle other than _M. geniohyoideus_ involved in the function of abducting the lower jaw of doves. Its size can be correlated especially well with feeding habits of the bird. Other birds that force their closed mandibles into fruit, wood or the earth and then forcibly open them, belong to groups possessing enlarged depressors. Contraction of the muscle pulls the postarticular (retroarticular) process upward with the resultant downward movement of that part of the mandible which is anterior to the articulation. Since there is no "gaping" in doves the muscle is only large enough to overcome the inherent tone of the relaxed adductor muscles. In some non-passerine species as well as in certain passerines the muscle also serves to raise the upper jaw by acting on the quadrate, which is capable of rotating vertically on its otic process. Especially in the gapers, where resistance is offered near the tip of the lower mandible, contraction of the muscle pulls the entire mandible dorsad thus forcing the jugal and palatal struts forward (Zusi, 1959:537-539). The action supplements that of _Mm. protractor pterygoidei et quadrati_ and is enhanced by anterior migration of the origin of _M. depressor mandibulae_. There is no lifting action involved in contraction of the depressor muscle in doves for two reasons--(A) the origin of the muscle is situated much too far posteriorly on the skull, and, more important, (B) the quadrate is not hinged for vertical movement. As will be discussed later, it moves only in a horizontal plane. ~_M. pseudotemporalis superficialis._~--Like _M. adductor mandibulae_, this muscle performs only the one function of adducting the lower mandible, and like _M. pseudotemporalis profundus_ it is a synergist of that muscle. ~_M. adductor mandibulae posterior._~--Although this muscle undoubtedly acts as an adductor of the lower mandible, I believe that, because of its disadvantageous insertion so near the articulation, its main function must be concerned with firming the mandible against the quadrate. This is to say that its function is partially that of a ligament. ~_M. protractor quadrati._~--When _M. protractor quadrati_ contracts, the quadrate bone is swung medially. This action, as mentioned previously, results in protraction of the upper jaw, and, as a consequence, its action supplements the action of _M. protractor pterygoidei_. CRANIAL OSTEOLOGY