730 Comparative Morphology of Chordates 



differ greatly even in the arrangement of the three main divisions of 

 the Class. For example, "Monotremata" has been counted as an 

 Order under Subclass Prototheria, and "Marsupialia" as an Order 

 under Subclass Metatheria. Or "Ditremata" may be listed as a Sub- 

 class under which Marsupialia and Placentalia appear as "Grades." 

 The subdivisions of Marsupialia and Placentalia are, on the whole, 

 fairly well stabilized. There is, however, some difference of opinion as 

 to the treatment of certain groups, especially the large and highly 

 diversified group of ungulates and the motley collection of animals 

 formerly herded together under "Edentata." The orders of one classi- 

 fication may appear as suborders in another. 



The type of differentiation of the teeth and the specialization of 

 the locomotor appendages are especially important as distinctive 

 characteristics, reflecting, as they must, the animal's diet, habitat, 

 and general mode of living. The number of teeth of each kind in the 

 heterodont dentition of a mammal is conveniently indicated by a 

 "dental formula," which is written in the form of a series of fractions, 

 each numerator being the number of teeth of a particular kind in the 

 upper jaw, and the denominator the number of corresponding teeth 

 in the lower jaw. The formula represents only one side of the jaws. 

 The kinds of teeth may be indicated by "i" (incisors), "c" (canines), 

 "p" (premolars), and "m" (molars). Premolars are teeth of the molar 

 type and are preceded by temporary "milk-molars"; molars are 

 posterior to the premolars and have no predecessors in the first denti- 

 tion. All teeth of the molar type (premolars and molars) are designated 

 as "cheek-teeth." The total number of teeth in the dentition is double 

 the sum of all the numbers in the formula. E.g., the formula for the 

 permanent dentition of the cat is 



9 /. 3 1 3 l\ 

 2 V3' C I' P 2' m lj 



= 30 



Some of the more typical or better-known members of each group 

 are mentioned. The statements about geographic distribution refer to 

 existing mammals only. The term "world-wide" is conveniently used 

 to indicate extensive distribution in both Eastern and Western Hemi- 

 spheres, but with such limitations as may be imposed by climatic or 

 other conditions. The statement that occurrence of Carnivora is 

 "world-wide" does not mean that lions are found at the North Pole. 

 Not all species of a group are to be found in every region of the terri- 

 tory specified as its area of distribution. The territory includes all 

 regions in which the group is represented, even if by only a single 

 species. 



