MAMMALIA. 461 
importance of correctly interpreting the dentition ofa mammal 
as far as possible at sight can hardly be over-estimated. 
The incisor and canine teeth remain more or less siniple throughout 
the majority of the Aammatta. In Ayrax, Galeofithecus and some 
rodents the incisors have their edges indented to form small cones, but 
these are exceptional. Again, the canines may resemble premolars in 
shape and may have more than one root (Pliohyrax, Erinaceus). The 
cheek-teeth or premolars and molars show infinite variety in shape and 
size according to the uses to which they are put. 
The complex types are derived from the more simple by the forma- 
tion of cusps or tubercles which may fuse to form ridges and crests. 
We may notice a few of the more important changes :— 
1. It is usually assumed that the earliest mammals had a homodont 
dentition like that of reptiles, each tooth being a simple cone. Those 
of the upper jaw fitted between those of the lower jaw, forning a ‘‘rat- 
trap” ‘arrangement, very efficient for seizing prey, but of little use for 
purposes of mastication. This first type is called a Aaplodon¢ dentition 
and is still found in the toothed whales (see Porpoise).. 
2. The next differentiation is the origin of small secondary cones 
upon the sides of each tooth, typically one on each side of the primary 
cone, though the whole margin may be serrated. These secondary” 
cones move upon those of the next tooth and considerably add to the 
‘‘tearing” and rending capacity of the teeth. Typically there is one 
cone on each side of the primary one, hence this type is known as the 
triconodont dentition. In the upper jaw the primary cone is known as 
the protocone, the anterior secondary one as the paracone and the 
posterior as the metacone. Those of the lower jaw are known as 
protoconid, paraconid and metaconid. 
3. In the next type the secondary cones move out of the same line 
as the main cone, those of the lower jaw moving inwards and those of 
the upper jaw outwards, The three cones or tubercles are now 
arranged in a triangle. The dental surface has no less than three 
interlocking rows of tubercles, the outer formed by the paracones and 
metacones, the middle by the protoconids, and the inner by the proto- 
cones with the paraconids and metaconids. This type is known as the 
tritubercular and is a very important one. It occurs in many modern 
mammals with little modification, such as certain /wsecttvora and Car- 
nivora, and is also very general amongst the mesozoic metatherian 
mammals and inodern Polyprotodontia. 
From the tritubercular type onwards we may trace three series. In 
one there is specialisation for a true carnivorous type producing the 
secodont or cutting dentition. In this the cones become connected by 
ridges which retain a sharp edge, acting as cutting organs. (Carndvora.) 
In the second the cones remain blunt and increase considerably 
in number. In later life their surfaces are ground away and there 
may further be important fusions forming blunt ridges. This is the 
bunodont series, found in herbivorous and omnivorous mammals. 
In the third the general tubercular character is retained though 
other cones may be added. This is probably to be traced to the reten- 
tion of a similar mode of nutrition and the examples are naturally to be 
