350 On the Evolution of the Vertebrata, [April, 
postzygapophysis upwards and outwards, so that the vertical sec- 
tion 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 
Vertebrata, 
Thirdly, as regards the dentition. Of the two types of Mono- 
tremata, the Tachyglosside and the Platypodide, 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 Theromorphous 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. Inthe marsupial order we have a great range of dental 
structure, which almost epitomizes that of the Monodelph orders. 
The dentition of the carnivorous forms is creodont; of the kan- 
garoos is perissodactyle, and that of the wombats is rodent. 
Other forms repeat the Insectivora. I therefore consider the pla- 
cental 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 three-sided 
prism, with tritubercular apex, which alternates with that of the 
opposite jaw. Fourth, development of a heel projecting from 
the posterior base of the lower jaw, which meets the crown of 
the superior, forming a tubercular-sectorial inferior molar. From 
this stage the carnivorous and sectorial dentition is derived, the 
tritubercular type being retained. Fifth, the development of a 
posterior inner cusp of the superior molar and the elevation of 
the heel of the inferior molar, with the loss of the anterior inner 
cusp, Thus the molars become quadritubercular, and opposite. 
This is the type of many of the Taxeopoda, including the Quad- 
rumana and Insectivora as well as the inferior Diplarthra. The 
higher Taxeopoda (Hyracoidea) and Diplarthra add various com- 
plexities. Thus the tubercles become flattened and then concave, 
so as to form Vs in the section produced by wearing, or they are 
_ Joined by cross-folds, forming various patterns. In the Probos- 
cidia they become multiplied so as to produce numerous cross- 
crests. 
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