28 QUADRATE AND INCUS CHAP. 
matter. But the hall-mark of truth is not always simplicity ; 
indeed the converse appears to be frequently the case. And 
on the whole this view does not commend itself ‘to zoologists 
at present. For it must be borne in mind that the lower jaw of 
the mammal is not the precise equivalent of that of the reptiles. 
Apart from the membrane bones, which may be collectively the 
equivalents of the dentary of the mammal, there is the cartilaginous 
articular bone to be considered, which forms the connexion 
between the rest of the jaw and the quadrate in reptiles. Even 
in the Anomodontia, whose relations to the Mammalha are con- 
sidered elsewhere, there is this bone. But in these reptiles the 
articular bone articulates not only with the quadrate, but also to 
a large extent with the squamosal, the quadrate shrinking in 
size and developing processes which give to it very much the 
look of either the incus or the malleus of the mammahan ear. 
In fact it seems on the whole to fit im with the views of the 
majority, as well as with a fair interpretation of the facts of 
embryology, to consider that the chain of ear bones in the 
mammal is not the equivalent of the columella of the reptile, 
but that 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 reptile 
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 hearing, 
Another important and diagnostic feature in the mammalian 
skull is that the first vertebra of the vertebral column always 
articulates with two separate occipital condyles, which are borne 
by the exoecipital 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 here 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. 
