106 MAMMALIAN DESCENT. [LECT. IV. 



grinding action not taking place ; thus these muscles- 

 do not require to have much force. 



I have not only followed the development of the skull 

 in the toothless Pangolin but also in the tooth-bearing 

 Armadillo. In the embryo of the Pangolin the first, or 

 cartilaginous skull (chondrocranium) is feeble, but in the 

 Armadillo it is as well formed as in the Crocodile, 

 or even as in the Frog or Toad ; this is soon crusted 

 over with radiating films of superficial bone. The 

 cartilage, as in us, also becomes bony, then the super- 

 ficial and the deep skeletal elements become largely 

 fused together to form a sort of ivory-box, with a 

 front extension, forming the organ of smell, and with 

 recesses, below, supporting the parts of the mouth and 

 throat. As no tooth-pulps are formed in the Pangolin, 

 the bones of the jaws are not hollowed out, but are very 

 similar to what is seen in the embryo of the Bird, 

 especially of one of the lower kinds the Ostrich, &c. 

 This flatness of the jaw-bones makes the skull, even at 

 this early stage, very dissimilar to that of the other 

 Mammalia, where the bones swell out with their growing 

 tooth-pulps and teeth. 



I have already mentioned that a change, essentially 

 equivalent to metamorphosis, soon takes place in this 

 little skull. This is mainly seen in the inferior arches 

 forming the skeleton of the mouth and throat, knd is 

 the last degree of specialisation undergone by a Verte- 

 brate in this part of its organisation. I have already 





