OSSEOUS SYSTEM OF AVES. 55 



bones, 26 and 27, which become blended together in young 

 Struthionidce before the confluence with the maxillary is complete, 

 extend backward in all adult birds, usually in a straight line, 

 from the maxillary to the tympanic. In the Cassowary the zygo- 

 matic arch presents a slight expanse and outward bend of the 

 squamosal ; in Didus it shows a slight downward as well as 

 outward bend. In some Parrots and in Hornbills (^Buceros), 

 the malo-squamosal zygomatic style, fig. 30, Z, has a moveable 

 cotyloid joint at both ends ; in some Caprimidgi it is anchylosed 

 at both ends. 



In the birds in which the upper mandible is moveable, either, 

 as in Parrots, by articulation, or as in many other birds by 

 flexibility of the nasal process of the premaxillary, the movements 

 of the tympanic, to and fro, upon its proximal joint, are transferred 

 by the zygoma to the maxillary, and by the pterygoid to the 

 palatine : and thus by the forward rotation of the tympanic the 

 upper jaw is raised, at the same time that the lower jaw by the 

 action of the digastricus may be depressed. 



Before anatomy had reached its homological phase, ornitho- 

 tomists called the zygomatic styles ^ ossa communicantia,' and the 

 pterygoids ^ ossa homoidea, seu interarticularia ; ' the folloAving 

 bone was termed ^ os quadratum.' 



In the tympanic, figs. 28, 31, 28, are to be noticed the ^ mastoid ' 

 and ^ mandibular ' ends, and the intermediate body giving attach- 

 ment at its back part to the ear-drum, and sending from its fore 

 part the ' orbital ' process. The mastoid articular end is obliquely 

 extended from behind forward and outward: the body slightly 

 contracts below ; then expands and becomes triedral at the set- 

 ting off of the broad compressed angular orbital process : below 

 this process the mandibular end is much expanded, chiefly trans- 

 versely : it presents two articular surfaces ; the outer one, elon- 

 gate or reniform, partly concave, partly convex ; the inner one 

 a shorter elliptic or oblong convexity ; the intermediate non- 

 articular tract varies in diflerent birds. On the outer side of the 

 mandibular end is a hemispheric articular cavity for the ^squa- 

 mosal.' In most birds the mastoid condyle is divided into two, 

 the inner and posterior encroaching upon the paroccipital, and 

 shomng, in an interesting way, the course of retrogression of the 

 tympano-mandibular arch from the fish to the Avarmblooded ovipara. 

 Most Cur sores and Rasores, Apteryx^^ Pezus, Rhynchotis, have 

 but one condyle, as in Lizards. The ear-drum is attached to the 

 back part of the pedicle obliquely from its outer margin above to 

 ' XVI-. torn. iii. pi. 39, figs. 8, 9, and xxiv, 



