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Show ~30 ON THE STRUCTURE OF 'filE SKULL. and Reptiles, and is never known to articulate, b~ a moveable joint, with the malleus, which, as we have s.een, IS the representative of the os articulare of the mandible of the lower Vertebrata. It is impossible, therefore, that the quadrate bone should be the homologue of the tympanic of Marnmalia. On the other hand, it corresponds altogether with the ~uadrate bone ~f Fishes, which is united in like manner With the pterygoid arcade, and is similarly connected by a moveable joint with the articular piece of the mandible ; and this quadrate bone of Fishes is, I have endeavoured to show, the homologue of the incus of the Mammalia. I make no question that, as Reichert long ago asserted, the Bird's os quadratum an~, th:refore, that of the Reptile, is the equivalent of the Mam1uahan ~ncus. It is difficult to understand how any doubt can be entertained as to the bone which is the homologue of the Mammalian squamosal in Birds. Lying above the tympanic cavity, between the parietal, frontal, and periotic bones, is a 1uembrane bone (Sq, Fig. 92, A) which corresponds with the 1\Iammalian squamosal and with no other bone in the Mammalian skull. Bu' t if this be the Bird's squamosal, there is no difficulty in determining that of any Reptile, the Orocodilia, Lacertilia, Chelonia, and Ophidia all presenting a bone in a similar position. It is this bone which, in most Ophidia (Fig. 92, 0 ), Fig. 93. \n Fig. 93.-The skull of a Lizard (Cyclodus).-D D, Dentary piece of the lower jaw; Qn, Os quadratum ; Sq, Squamosal. carries the quadratum. as on a lever ; but, as Rathke has well shown, the final position of the quadratum is a result of develop- 'l'HE SKULLS OF REP'l'ILIA AND AVES. 231 mental modification, the proximal end of that bone b · 0' ori~in~lly in Ophidia, as in other Reptiles, applied toe~~~ periOtic capsule. The palato-maxillary apparatus presents a considerable diversity of structure in Reptiles and Birds. In all Birds and · R 'l ' ln most epti es, the pterygoid and the quadrate bones are more or less closely connected, but in the Crocodiles and Ohamooleons they are separated. In Orocodiles and Oltelonia and in the e~tinct Plesiosauria, the quadrate bone is immo:eably united w1~h tdhe .skull, and the other f~cial Lones are firmly and fixedly un1te w1th one another and with the cranium. In most Bird , and Lacertilia, on the contrary, the quadrate bone is n1oveably Fig. 94. A B I .EO Fig. 94.-Viedw sB o.f o'1n' e ,h alf of the palatine surface of the skull of. A a L · ..1 (C 1 d ) 1 . , , ' 1za1 u yc~o u.s , an , a mt e (Chelone m~das). N 1, the posterior nasal apertnre, articulated with the skull, and its motion may be communicated by the pterygoid, the quadrato-jugal and the jugal bones to the fore-part of th~ face. This mobility reaches its maximum, on the one hand, In such birds as the Parrots, in which the beak and fore-part of the basi-facial axis arc united by a sort of hinge |