No. 2.] THE HARD PARTS OF THE MAMMALIA. 225 
terior base of the lower jaw, which meets the crown of the supe- 
rior, forming a tuberculo-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 
complexities. Thus the tubercles become flattened and then 
concave, so as to form V’s in the section produced by wearing ; 
or they are joined by cross-folds, forming various patterns. In 
the Proboscidia the latter become multiplied so as to produce 
numerous cross-crests. 
The dentition of some of the Sirenia is like that of some of 
the Ungulata, especially of the suilline group, while in others 
the teeth consist of cylinders. In the Cetacea the molars of the 
oldest (Eocene and Miocene) types are but two-rooted and com- 
pressed, having much the form of the premolars of other Mam- 
malia. In existing forms a few have simple conical teeth, while 
in a considerable number teeth are entirely wanting. 
Of the two types of Monotremata, the Tachyglossidz and the 
Ornithorhynchide, 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 (the 
Multituberculata) 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 carnivorous forms is creodont ; that of the kan- 
garoos is perissodactyle, and that of the wombats is rodent. 
Other forms repeat the Insectivora. I consider the placental 
series especially, as the evolution has been similar in both cases. 
In ascertaining the mechanical causes of these types of teeth, 
the first essential is to ascertain the character of their move- 
ments. The following table gives a summary of them :— 
