395 



tervenes between the petrosal and squamosal bones above, and the 

 articular portion of the lower jaw below ; which articulates with the 

 pterygoid internally, and with the quadratojugal externally, which 

 gives attachment to a part of the tympanic membrane posteriorly, 

 and which is very generally termed the tympanic bone, from its 

 supposed homology with the bone so named in the Mammalia. The 

 resemblance to the tympanic bone, however, hardly extends beyond 

 its relation to the tympanic membrane ; for in no other of the par- 

 ticulars mentioned above do the connexions of the two bones cor- 

 respond. The tympanic of the mammal does not articulate with the 

 lower jaw, nor with the pterygoid*, nor with the jugal or quadrato- 

 jugal. On the other hand, if the connexions of the tympanic mem- 

 brane were sufficient to determine the point, not only the quadratum, 

 but the articular element of the lower jaw, and even some cranial 

 bones, must be regarded as tympanic f . 



Again, if we trace the modifications which the tympanic bone 

 undergoes in the mammalian series, we find that in those mammals, 

 such as Echidna and Ornithorhynchus, which approach nearest to 

 the Ovipara, and which should therefore furnish us with some hint 

 of the modifications to which the tympanic bone is destined in that 

 group, the bone, so far from increasing in size and importance, 

 and taking on some of the connexions which it exhibits in the 

 oviparous Vertebrata, absolutely diminishes and becomes rudi- 

 mentary, so that the vast bony capsule of the placental mammal 

 is reduced, in the monotreme, to a mere bony ring. 



But it is no less worthy of remark, that in these very same animals 

 the malleus and incus have attained dimensions out of all proportion 

 to those which they exhibit in other mammals, and that they even 

 contribute to the support of the tympanic membrane. 



So far, therefore, from being prepared by the study of those 

 Mammalia which most nearly approach the Ovipara, to find, in the 

 most highly organized of the latter, an immense os tympanicum, 

 with a vanishing malleus and incus, we are, on the contrary, led to 

 anticipate the disappearance of the tympanicum, and the further 

 enlargement of the ossicula auditus. Thus far the cautious appli- 

 cation of the method of gradations leads us, and leads us rightly 



* Though the pterygoid comes close to it in Monotremata. 

 t See Note III. 



