44 MAMMALIAN DESCENT. [Lect. II^ 



oval window {fenestra ovalis) that lies between the drum, 

 cavity and the vestibular part of the labyrinth of the 

 ear. That bone (the stapes) exists in Birds and Eeptiles, 

 but the other two, as such, do not. Also, in them, it 

 is not a stirrup, but a little column {columella). So it 

 is in these low mammals. 



We have then, in this curious piece of morphology, no 

 new structure, but a very new specialisation of an old 

 one. Whatever parts grow out of, or are attached to,, 

 the columella of the ovipara, are merely processes, or at 

 most, segments, of the " pharyngo-hyal " element of the- 

 tongue-arch, or uppermost piece of the arch. 



Thus, in mammals, by a curious horticultural process,, 

 so to speak, two new elements are added to the auditory 

 chain, namely, the incus and malleus. These parts, 

 so modified, are diagnostic of a mammal. Why they 

 should be correlated with mammary glands, and hair,. 

 I cannot say. 



I have yet to speak of the most remarkable part of 

 the skuU of the DucJcbill ; I refer to the composition of 

 its beak. Much as it resembles the beak of a duck, its 

 structure is widely different, yet the superficial bones- 

 are homologous, and not altogether dissimilar; these- 

 are the premaxillaries in front, the maxiUaries exter- 

 nally, the nasals above, and the palatines and pterygoids- 

 below. 



All these bones are peculiarly thin and lathy in the 

 young animal. They do not, as in the Duck, finish the 



