76 THE GENEALOGY OF THE VERTEBRATA. 



the tibia and radius, and their middle portions became 

 attenuated or disappear. 



Secondly, as regards the vertebrae. The mutual artic- 

 ulations (zygapophyses) in the Condylarthra are flat and 

 nearly horizontal. In higher forms, especially of the 

 ungulate series, they become curved, the posterior turn- 

 ing upwards and outwards, and the anterior embracing 

 them on the external side. In the higher Diplarthra this 

 curvature is followed by another curvature of the post- 

 zygapophysis upwards and outwards, so that the verti- 

 cal section of the face of this process is an S. Thus is 

 formed a very close and secure joint, such as is nowhere 

 seen in any other Vertebra ta. 



Thirdly, as regards the dentition. Of the two types 

 of Monotremata, the Tachyglossidse and thePlatypodidre. 

 the known genera of the former possess no teeth, and 

 the known genus of the latter possesses only a single 

 corneous epidermic grinder in each jaw. As the Thero- 

 morphous reptiles from which these are descended have 

 well developed teeth, their condition is evidently one of 

 degeneration, and we can look for well toothed forms of 

 Monotremata in the beds of the Triassic and Jurassic 

 periods. Perhaps some such are already known from 

 jaws and teeth. In the marsupial order we have a great 

 range of dental structure, which almost epitomizes that 

 of the Monodelph orders. The dentition of the car- 

 nivorous forms is creodont ; that of the kangaroos is 

 piM'issodactyle, and that of the wombats is rodent. 

 Other forms repeat the Insectivora. I therefore con- 

 sider the placental series especially. I have already 

 shown that the greater number of the types of this series 

 have derived the characters of their molar teeth from 

 the stages of the following succession. First a simple 

 cone or reptilian crown, alternating with that of the 

 other jaw. Second, a cone with lateral denticles. Third, 

 the denticles to the inner side of the crown forming a 



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