2 8 QUADRATE AND INCUS chap. 



matter. But the hall-mark of truth is not always simplicity ; 

 indeed the converse appears to be frequeutlj the case. And 

 on the whole this view does not commend itself to zoologists 

 at present. For it must he l)orne in mind that the lower jaw of 

 the manniial is not th(> precise equivalent of that of the reptiles. 

 Apart from the membrane bones, whicli may be collectively the 

 etpiiyalents of the dentary of the mamm;d, there is the cartilaginous 

 articular bone to l)e considered, which forms the connexion 

 between the rest of the jaw and the quadrate in reptiles. Even 

 in the Anomodontia, whose relations to tlie Mannualia are con- 

 sidered elsewhere, there is this bone. But in these reptiles the 

 articular Ijone articulates not only with the quadrate, but also to 

 a large extent with tlie squamosal, the quadrate shrinking in 

 size and developing processes which give to it very much the 

 look of either the incus or the ^nalleus of the mamnifdian ear. 

 In fact it seems on the whole to fit in with the views of the 

 majority, as well as with a lair interpretation of the facts of 

 embryology, to consider that the chain of ear bones in the 

 inauimal is not the e(|uiyalent of the columella of the reptile, 

 but tliat the stapes of the mammal is the columella, and that 

 the articulare is represented by the malleus and the quadrate 

 by the incus. It is very interesting to note this entire change 

 of function in the bones in question. Bones which in the re})tile 

 serve as a means of attachment of the lower jaw to the skull are 

 used in the mammal to convey the waves of sound from the 

 tympanum of the ear to the internal organ of liearing. 



Another important and diagnostic feature in the mammalian 

 skull is that the first vertebra of the verteliral column always 

 articulates with two separate occipital condyles, which are borne 

 1)y the exoccipital bones and formed mainly though not entirely 

 by them. Certain Anomodontia form the nearest approach to 

 the mammals in this particular. The two condyles of Amphibia 

 are purely exoccipital in origin. 



In the Mammalia, unlike what is found in lower Vertebrates 

 (but liere again the Anomodontia form at least a partial exception), 

 the jugal arch does not connect the face with the quadrate, for, 

 as already said, that bone does not exist, in the Sauropsidan 

 form, in mammals. This arch passes from the squamosal to the 

 maxillary, and has but one separate bone in addition to those 

 two, viz. the jugal or malar. 



