55 



small brains, and in some forms this organ was proportionally 

 less than in certain Eeptiles. There was a gradual increase in 

 the size of the brain during this period, and it is interesting to 

 find that this growth was mainly confined to the cerebral 

 hemispheres, or higher portion of the brain. In most grou})S 

 of mammals, the brain has gradually become more convoluted, 

 and thus increased in quality, as well as quantity. In some, 

 also, the cerebellum, and olfactory lobes, the lower parts of the 

 brain, have even diminished in size. In the long struggle for 

 existence during Tertiary time, the big brains won, then as 

 now ; and the increasing power thus gained rendered useless 

 many structures inherited from primitive ancestors, but no 

 longer adapted to new conditions. 



Another of the interesting changes in mammals during Ter- 

 tiary time was in the teeth, which were gradually modified 

 with other parts of the structure. The primitive form of tooth 

 was clearly a cone, and all others are derived from this. All 

 classes of vertebrates below mammals, namely. Fishes, Amphi- 

 bians, Eeptiles, and Birds, have conical teeth, if any, or some 

 simple modification of this form. The Edentates and Ceta- 

 ceans with teeth retain this type, except the Zeuglodonts, which 

 approach the dentition of aquatic Carnivores. In the higher 

 mammals, the incisors and canines retain the conical shape, and 

 the premolars have only in part been transformed. The latter 

 gradually change to the more complicated molar pattern, and 

 hence are not reduced molars, but transition forms from 

 the cone to more complex types. Most of the early Tertiary 

 mammals had forty-four teeth, and in the oldest forms the 

 premolars were all unlike the molars ; while the crowns were 

 short, covered with enamel, and without cement. Each stage 

 of progress in the differentiation of the animal was, as a rule, 

 marked by a change in the teeth ; one of the most common 

 being the transfer, in form at least, of a premolar to the molar 

 series, and a gradual lengthening of the crown. Hence, it is 

 often easy to decide from a fragment of a jaw, to what horizon 

 of the Tertiary it belongs. The fossil Horses of this period. 



