644 



MAMMALIA 



[CH. 



undergone profound modifications in accordance with the different 

 habits assumed by different members of the class, and are one of 

 the principal features on which its division into orders is based. 

 From a study of the dentition of living Mammals the con- 

 clusion is arrived at that the typical number of teeth, that is to 



--7 



FIG. 318. Four diagrams to illustrate the evolution of the ear-bones in Mam- 

 malia. The diagrams represent the bones of the back of the lower jaw 

 viewed from the inner side. The tympanic membrane is cross hatched 

 and cartilage bones are covered with small circles, whilst membrane bones 

 are left unshaded. 



A. Condition in early Theromorphous Reptile. The articular and quadrate 

 are large, and the dentary does not meet the squamosal. 



B. Condition in later Theromorphous Eeptile. The dentary has met the 

 squamosal, and the quadrate and articular are reduced in size. 



C. Condition in hypothetical form, the link between Theromorpha and Mam- 

 malia. The supra-angular has begun to extend along the border of the 

 tympanic membrane. 



D. Condition in primitive Mammalia (Prototheria). 



1. Squamosal. 2. Dentary. 3. Supra-angular. 4. Articular (= Malleus). 

 5. Quadrate (= Incus). 6. Columella auris ( = Stapes). 7. Tympanic 

 membrane. 



say, the number which the common ancestral form possessed, may 

 be estimated at 44, i.e., 11 on each side of each jaw, made up of three 

 incisors, one canine, four premolars, and three molars. This fact is 



Q "I A Q 



expressed by the formula , where the upper line denotes 



o . 1 . 4 . o 



the teeth on each side of the upper jaw and the lower line those on 

 each side of the lower jaw. 



