FHYLOGENY. 135 



it is a flat joint or not tongued or grooved. In most 

 of the Carnivora, in a few Glires, and in all Diplar- 

 thra, it is deeply tongued and grooved, forming a more 

 perfect and stronger joint than in the other orders, 

 where the surfaces of the tibia and astragalus are flat. 

 (D) In the highest forms of the Rodentia and Diplar 

 thra the fibula and ulna become more or less coossified 

 with the tibia and radius, and their middle portions 

 become attenuated or disappear. 



Secondly, as regards the vertebrae. The mutual 

 articulations (zygapophyses) in the Condylarthra have 

 flat and nearly horizontal surfaces. In higher forms, 

 especially of the ungulate series, they become curved, 

 the posterior turning upward and outward, and the an- 

 terior embracing them on the external side. In the 

 higher Diplarthra this curvature is followed by another 

 curvature of the postzygapophysis upward and out- 

 ward, so that the vertical section of the face of this 

 process is an S. Thus is formed a very close and se- 

 cure joint, such as is nowhere seen in any other Verte- 

 brata. 



Thirdly, as regards the dentition. Of the two types 

 of Monotremata, the Tachyglossus, and the Platy- 

 pus, the known genera of the former possess no teeth, 

 and the known genus of the latter possesses only a 

 single corneous epidermic grinder succeeding two de- 

 ciduous molars, like those of certain extinct forms, in 

 each jaw. As the theromorous reptiles from which 

 these are descended have well-developed teeth, their 

 condition is evidently one of degeneration. We prob- 

 ably have their ancestors in the Multituberculata, 

 which range from Triassic to lower Eocene time in 

 both hemispheres. In the marsupial order we have a 

 great range of dental structure, which almost epito- 



