36 MAMMALIAN DESCENT. [Lect. IL 



divided into segments corresponding with the costal 

 cinctures, or arches of the chest ; this is diagnostic of the 

 mammal, yet it begins in certain Lizards, e.g., the 

 Chameleon. That which makes the vertebra of a 

 mammal differ from that of the higher oviparous t3rpes 

 is the development of the flat epiphyses or separate 

 bony plates on its body or centrum. 



Now these are nearly absent in the Prototheria. 

 Albrecht and Huxley, however, have found them in the 

 vertebrae of the tail in OrnitJiorhynchus. This fact, 

 again, is very instructive — the Monotreme is feeling its 

 way upwards to the higher platform on which we stand. 



With regard to the skull, there is much of the deepest 

 interest to the evolutionist, even in our present partial 

 knowledge of its development. The Ornithorhynclnis is 

 ])y far the most primitive type ; the Echidna has a huge 

 brain for so foolish a creature, and it comes very near 

 the Ant-eaters, proper, in many of its cranial characters. 

 When I come to the Edentata, the group which contains 

 the Ant-eaters, I shall refer to this fact again. At 

 present I shall confine myself to the DuchhiU. That 

 Avhich strikes the eye at once is the very amphibian look 

 of the whole structure of the skull ; it is like that of 

 some strange Dipnoan or Salamandrian just under- 

 going transformation. 



We, like our fellow- vertebrates, have at first a carti- 

 lagfinous cranium that forms the foundation of the 

 finished ivory casket which, in the adult, so safely holds 



