GENERAL STRUCTURE. 1 257 



as we shall see in the sequel, is the only existing heterodont 

 Mammal with more than four true molars. In the second group 

 the dentition is of a very different type ; the lower jaw having a 

 single pair of curved and chisel-like incisors, separated by an 

 interval from the cheek-teeth, which are characterised by the 

 presence of one or more longitudinal grooves. The serial position 

 of this group, which has been named Multituberculata, is still 

 open to doubt, but not improbably it may indicate an extinct order 

 of Prototheria. 



Throughout the greater part of the Cretaceous period our know- 

 ledge of Mammalian life is a blank, doubtless owing to the circum- 

 stance that the greater portion of the Cretaceous beds, such as the 

 Chalk, is of purely marine origin ; but evidence has recently been 

 obtained of the existence of a Mammalian fauna in the topmost 

 Cretaceous of North America allied to that of the Jurassic. 



With the dawn of the Tertiary period, which may be regarded as 

 the first commencement of the present order of nature, we meet, 

 however, with an abundant Mammalian fauna, containing repre- 

 sentatives of nearly all the existing orders, but also including several 

 subordinal types now totally passed away, some of which are of 

 extreme interest to the zoologist as connecting together more 

 or less completely groups which are now widely separated. In 

 the Eocene we are indebted to the palaeontologists of the New 

 World for most of our knowledge of these primitive connecting 

 links ; but we can only afford space to notice very briefly some of 

 the more interesting groups. 



Of Carnivorous types there is a group of Eocene genera known 

 as the Creodonta, remarkable, among other features, for their 

 generalised dentition, which has resemblances to that of the Poly- 

 protodont Marsupialia and Insectivora, and also to that of the 

 modern Carnivora, of which these forms were probably the ances- 

 tors. We thus have indications how the Carnivora of the present 

 day may have been gradually evolved from a Marsupial type by 

 means of forms more or less nearly related to the Insectivora of 

 the present epoch. In another direction the Eocene has afforded 

 evidence of a transition from the Insectivorous type towards that 

 of the Lemuroid Primates, and we may thus readily conceive how 

 the higher members of the latter order may likewise trace back 

 their origin to the same primitive stock. 



At the present day no orders of Mammals appear more sharply 

 defined from one another than the Carnivores and the Ungulates. 

 In the Eocene, however, we meet with a group of primitive Un- 

 gulates, known as the Condylarthra, presenting such remarkable 

 resemblances to the primitive Carnivores, that we are led to the 

 conclusion that the Ungulates are probably another branch derived 



