MAMMALIA. 461 



importance of correctly interpreting the dentition of a mammal 

 as far as possible at sight can hardly be over-estimated. 



The incisor and canine teeth remain more or less simple throughout 

 the majority of the Mammalia. In Hyrax^ Galeopithecus and some 

 rodents the incisors have their eds^es 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, Erinaceiis). 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, forming a " rat- 

 trap " arrangement, very efficient for seizing prey, but of little use for 

 purposes of mastication. This first type is called a haplodout 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 

 t7'icouodont 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 

 trituberculai' and is a very important one. It occurs in many modern 

 mammals with little modification, such as certain Insectivora and Car- 

 nivora, and is also very general amongst the mesozoic metatherian 

 mammals and modern 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. ( Carnivora. ) 



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 

 bzuiodont 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 



