44 MAMMALIAN DESCENT. [Lect. II^ 



oval window [fenestra ovalis) tliat lies between the drum 

 cavity and tlie vestibular part of the labyrinth of the 

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

 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 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 



