44 MAMMALIAN DESCENT. [LECT. IT. 



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 Reptiles, 

 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 r 

 I cannot say. 



I have yet to speak of the most remarkable part of 

 the skull of the Duckbill ; 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 maxillaries 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 



