108 MAMMALIAN DESCENT. [Lect. IV. 



arch of tlie face, and, as low down among the types as 

 the Newt or the Salamander, is used for a 7ieio physio- 

 logical purpose. But the addition of two new segments 

 to the middle ear-chain in all mammals takes place by 

 the arrest and modification of the hinge-piece of the 

 upper jaw, and the corresponding part of the lower jaw. 

 Thus, the amputated lower jaw has to fasten itself to a 

 new swinging point ; that, of course, is further forwards 

 than in Birds, Eeptiles, and Amphibia, in which this 

 transformation does not take place. 



In the smallest Pangolin embryo a little trowel of 

 bone is formed over the temporal region, just as in a 

 Chick ; and, as in that bird, the handle of this small 

 trowel projects forwards and downwards over the cheek. 



This well-known zygomatic i?rocess of the temporal 

 bone is a mere spur for muscular attachment in the Fowl ; 

 but in the Pangolin it flattens out at the end, and 

 acquires a small patch of cartilage beneath. This is the 

 articular cartilage of the "glenoid cavity" (or socket for 

 the lower jaw); the "ramus" or lower jaw reaches up 

 to this part, covered here with articular cartilage ; a 

 meniscus of fibro-cartilage intervenes to serve as a 

 " buffer " in the act of eatins^. 



In this embryo most of the rudimentary lower jaw 

 is cartilage, but a thin film of bone is forming on it ; 

 yet the coronoid, articular, and angular regions are 

 all cartilaginous. This is the superficial or secondary 

 mandible ; but inside it there is a solid rod of cartilage 



